Qt Creator seems to try to use QTC_CLANGD_COMPLETION_RESULTS environment variable to pass the value for clangd --limit-results, but this does not seem to work for some reason. Instead 0 is always passed when looking at the command line through ps -Ao command | grep limit-results.

Qt Creator does show this environment variable in Build Environment, so it does seem to get through in general, just not used for clangd.