See my comment here.

It seems that some dynamic_cast usage and dependency on RTTI has snuck into Qt over time. I find this surprising as I haven't seen this requirement change listed anywhere in Qt6 documentation / porting information. Since the commit which caused the assert I initially noticed checks __cpp_rtti, it seems like Qt does still intend to support non-RTTI builds.

Should the RTTI usages pointed out by the compiler warnings be considered bugs, then?
Warnings can be seen here https://bugreports.qt.io/browse/QTBUG-98905?focusedCommentId=657946&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-657946

If the RTTI usage could be limited to Qt-internal usage that would be fine, but IMO it's a kind of large breaking API change if you require users of Qt to start enabling RTTI, now.