To be clear, it's not that 32-bit codecs are going away, it's that all 32-bit software is going away. The article is fairly sparse and doesn't go into detail about the change or explain why it is occurring. You may have read Apple's article about 'legacy media' being phased out in the next version of macOS or even been alerted by an error message in the new version of FCPX.
Thoughts on 32-bit codecs being phased out in macOS
