For quite a few years, builds for newer macOS versions come with 8.6 while builds for very *old* macOS versions *may* have 8.4. In any case, it's best to check the available Tcl version on load in your plugin / code and issue a warning if it's not supported.
This is something that should probably be added to the platform specific readme:
https://github.com/pure-data/pure-data/blob/master/mac/README.txt
Dear all
I?ve looked in the codebase and the list and I cannot find what tcl/tk I can be guaranteed for Pd on MacOS Linux and Windows? should I check in my tcl code if I need 8.5 (for dict support)
Thanks
p