Why would an app still use QuickTime over the newer and non-deprecated AV Foundation? Because AV Foundation does not support third-party codecs. But it affects video software the most because the only reason for an app to still be 32-bit in 2018 is because it uses QuickTime. 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
