QtCore in Version 6 exports some zlib associated functions, but the most important ones like z_gzopen are not exported. This makes Qt6 unusable. In total, 34 methods are not exported that were exported in Qt5:

Exported methods in Qt6 are (50 methods in total):
6175 (0x181f), (0x), z_zlibVersion, 0x00300aa0, None
6126 (0x17ee), (0x), z_adler32, 0x00300ef0, None
6127 (0x17ef), (0x), z_adler32_combine, 0x00300f00, None
6128 (0x17f0), (0x), z_adler32_z, 0x00300b00, None
6129 (0x17f1), (0x), z_compress, 0x002f8300, None
6130 (0x17f2), (0x), z_compress2, 0x002f8200, None
6131 (0x17f3), (0x), z_compressBound, 0x002f8320, None
6132 (0x17f4), (0x), z_crc32, 0x003014f0, None
6133 (0x17f5), (0x), z_crc32_combine, 0x00301560, None
6134 (0x17f6), (0x), z_crc32_combine_gen, 0x003015d0, None
6135 (0x17f7), (0x), z_crc32_combine_op, 0x003015e0, None
6136 (0x17f8), (0x), z_crc32_z, 0x00301100, None
6137 (0x17f9), (0x), z_deflate, 0x002fa230, None
6138 (0x17fa), (0x), z_deflateBound, 0x002fa0c0, None
6139 (0x17fb), (0x), z_deflateCopy, 0x002fbb10, None
6140 (0x17fc), (0x), z_deflateEnd, 0x002fb730, None
6141 (0x17fd), (0x), z_deflateGetDictionary, 0x002f9c50, None
6142 (0x17fe), (0x), z_deflateInit2_, 0x002fb7e0, None
6143 (0x17ff), (0x), z_deflateInit_, 0x002fbad0, None
6144 (0x1800), (0x), z_deflateParams, 0x002fb570, None
6145 (0x1801), (0x), z_deflatePending, 0x002f9f10, None
6146 (0x1802), (0x), z_deflatePrime, 0x002f9f70, None
6147 (0x1803), (0x), z_deflateReset, 0x002f9df0, None
6148 (0x1804), (0x), z_deflateResetKeep, 0x002f9d00, None
6149 (0x1805), (0x), z_deflateSetDictionary, 0x002f9a30, None
6150 (0x1806), (0x), z_deflateSetHeader, 0x002f9ec0, None
6151 (0x1807), (0x), z_deflateTune, 0x002fa060, None
6152 (0x1808), (0x), z_get_crc_table, 0x003010f0, None
6153 (0x1809), (0x), z_inflate, 0x002fc220, None
6154 (0x180a), (0x), z_inflateCodesUsed, 0x002fe6d0, None
6155 (0x180b), (0x), z_inflateCopy, 0x002fe310, None
6156 (0x180c), (0x), z_inflateEnd, 0x002fde10, None
6157 (0x180d), (0x), z_inflateGetDictionary, 0x002fde70, None
6158 (0x180e), (0x), z_inflateGetHeader, 0x002fdfc0, None
6159 (0x180f), (0x), z_inflateInit2_, 0x002fc060, None
6160 (0x1810), (0x), z_inflateInit_, 0x002fc170, None
6161 (0x1811), (0x), z_inflateMark, 0x002fe650, None
6162 (0x1812), (0x), z_inflatePrime, 0x002fc180, None
6163 (0x1813), (0x), z_inflateReset, 0x002fbf40, None
6164 (0x1814), (0x), z_inflateReset2, 0x002fbfa0, None
6165 (0x1815), (0x), z_inflateResetKeep, 0x002fbe90, None
6166 (0x1816), (0x), z_inflateSetDictionary, 0x002fdf20, None
6167 (0x1817), (0x), z_inflateSync, 0x002fe030, None
6168 (0x1818), (0x), z_inflateSyncPoint, 0x002fe2b0, None
6169 (0x1819), (0x), z_inflateUndermine, 0x002fe570, None
6170 (0x181a), (0x), z_inflateValidate, 0x002fe5d0, None
6171 (0x181b), (0x), z_uncompress, 0x00300a80, None
6172 (0x181c), (0x), z_uncompress2, 0x00300930, None
6173 (0x181d), (0x), z_zError, 0x00300ac0, None
6174 (0x181e), (0x), z_zlibCompileFlags, 0x00300ab0, None

But it should export (looking into Qt5, 84 in total):

6415 (0x190f), (0x), z_zlibVersion, 0x00073a20, None
6332 (0x18bc), (0x), z_adler32, 0x00063130, None
6333 (0x18bd), (0x), z_adler32_combine, 0x00063140, None
6334 (0x18be), (0x), z_adler32_z, 0x00062aa0, None
6335 (0x18bf), (0x), z_compress, 0x00063390, None
6336 (0x18c0), (0x), z_compress2, 0x000632a0, None
6337 (0x18c1), (0x), z_compressBound, 0x000633b0, None
6338 (0x18c2), (0x), z_crc32, 0x00063a40, None
6339 (0x18c3), (0x), z_crc32_combine, 0x00063a60, None
6340 (0x18c4), (0x), z_crc32_z, 0x00063a20, None
6341 (0x18c5), (0x), z_deflate, 0x000677b0, None
6342 (0x18c6), (0x), z_deflateBound, 0x00067630, None
6343 (0x18c7), (0x), z_deflateCopy, 0x00067ff0, None
6344 (0x18c8), (0x), z_deflateEnd, 0x00067840, None
6345 (0x18c9), (0x), z_deflateGetDictionary, 0x00066ce0, None
6346 (0x18ca), (0x), z_deflateInit2_, 0x00067950, None
6347 (0x18cb), (0x), z_deflateInit_, 0x00067d00, None
6348 (0x18cc), (0x), z_deflateParams, 0x00067330, None
6349 (0x18cd), (0x), z_deflatePending, 0x00067140, None
6350 (0x18ce), (0x), z_deflatePrime, 0x000671d0, None
6351 (0x18cf), (0x), z_deflateReset, 0x00066ed0, None
6352 (0x18d0), (0x), z_deflateResetKeep, 0x00066db0, None
6353 (0x18d1), (0x), z_deflateSetDictionary, 0x00066a40, None
6354 (0x18d2), (0x), z_deflateSetHeader, 0x000670b0, None
6355 (0x18d3), (0x), z_deflateTune, 0x000675a0, None
6356 (0x18d4), (0x), z_get_crc_table, 0x00063a10, None
6357 (0x18d5), (0x), z_gzbuffer, 0x00069010, None
6358 (0x18d6), (0x), z_gzclearerr, 0x00069570, None
6359 (0x18d7), (0x), z_gzclose, 0x00068270, None
6360 (0x18d8), (0x), z_gzclose_r, 0x0006a5c0, None
6361 (0x18d9), (0x), z_gzclose_w, 0x0006b770, None
6362 (0x18da), (0x), z_gzdirect, 0x0006a580, None
6363 (0x18db), (0x), z_gzdopen, 0x000688b0, None
6364 (0x18dc), (0x), z_gzeof, 0x000694f0, None
6365 (0x18dd), (0x), z_gzerror, 0x00069510, None
6366 (0x18de), (0x), z_gzflush, 0x0006b550, None
6367 (0x18df), (0x), z_gzfread, 0x00069f40, None
6368 (0x18e0), (0x), z_gzfwrite, 0x0006ad30, None
6369 (0x18e1), (0x), z_gzgetc, 0x0006a060, None
6370 (0x18e2), (0x), z_gzgetc_, 0x0006a150, None
6371 (0x18e3), (0x), z_gzgets, 0x0006a3e0, None
6372 (0x18e4), (0x), z_gzoffset, 0x000694a0, None
6373 (0x18e5), (0x), z_gzoffset64, 0x00069450, None
6374 (0x18e6), (0x), z_gzopen, 0x000682b0, None
6375 (0x18e7), (0x), z_gzopen64, 0x000685b0, None
6376 (0x18e8), (0x), z_gzopen_w, 0x00068ce0, None
6377 (0x18e9), (0x), z_gzprintf, 0x0006b370, None
6378 (0x18ea), (0x), z_gzputc, 0x0006aea0, None
6379 (0x18eb), (0x), z_gzputs, 0x0006b090, None
6380 (0x18ec), (0x), z_gzread, 0x00069e40, None
6381 (0x18ed), (0x), z_gzrewind, 0x00069050, None
6382 (0x18ee), (0x), z_gzseek, 0x00069280, None
6383 (0x18ef), (0x), z_gzseek64, 0x00069110, None
6384 (0x18f0), (0x), z_gzsetparams, 0x0006b630, None
6385 (0x18f1), (0x), z_gztell, 0x00069420, None
6386 (0x18f2), (0x), z_gztell64, 0x000693f0, None
6387 (0x18f3), (0x), z_gzungetc, 0x0006a240, None
6388 (0x18f4), (0x), z_gzvprintf, 0x0006b1a0, None
6389 (0x18f5), (0x), z_gzwrite, 0x0006ac10, None
6390 (0x18f6), (0x), z_inflate, 0x0006def0, None
6391 (0x18f7), (0x), z_inflateBack, 0x0006b9d0, None
6392 (0x18f8), (0x), z_inflateBackEnd, 0x0006cb80, None
6393 (0x18f9), (0x), z_inflateBackInit_, 0x0006b8d0, None
6394 (0x18fa), (0x), z_inflateCodesUsed, 0x00070630, None
6395 (0x18fb), (0x), z_inflateCopy, 0x00070280, None
6396 (0x18fc), (0x), z_inflateEnd, 0x0006fd50, None
6397 (0x18fd), (0x), z_inflateGetDictionary, 0x0006fdd0, None
6398 (0x18fe), (0x), z_inflateGetHeader, 0x0006ff40, None
6399 (0x18ff), (0x), z_inflateInit2_, 0x0006dc30, None
6400 (0x1900), (0x), z_inflateInit_, 0x0006dd30, None
6401 (0x1901), (0x), z_inflateMark, 0x000705c0, None
6402 (0x1902), (0x), z_inflatePrime, 0x0006de60, None
6403 (0x1903), (0x), z_inflateReset, 0x0006daa0, None
6404 (0x1904), (0x), z_inflateReset2, 0x0006db70, None
6405 (0x1905), (0x), z_inflateResetKeep, 0x0006d9e0, None
6406 (0x1906), (0x), z_inflateSetDictionary, 0x0006fe70, None
6407 (0x1907), (0x), z_inflateSync, 0x0006ffa0, None
6408 (0x1908), (0x), z_inflateSyncPoint, 0x00070220, None
6409 (0x1909), (0x), z_inflateUndermine, 0x00070510, None
6410 (0x190a), (0x), z_inflateValidate, 0x00070560, None
6411 (0x190b), (0x), z_uncompress, 0x00073a00, None
6412 (0x190c), (0x), z_uncompress2, 0x000738b0, None
6413 (0x190d), (0x), z_zError, 0x00073a40, None
6414 (0x190e), (0x), z_zlibCompileFlags, 0x00073a30, None