Apple has now confirmed that macOS 10.15 won’t support 32-bit software, which includes QuickTime 7 and all media formats and codecs relying on it. Mojave currently includes legacy support for QuickTime 7, so if you have old media files which rely on that and its codecs, now is the time to convert those to formats which will be supported in macOS 10.15.
Apple has released updates to iMovie, Final Cut Pro, Motion and Compressor which “detect media files that may be incompatible with future versions of macOS after Mojave and convert them to compatible formats.” You should upgrade to those and check your old media files in the coming weeks. Those files which use formats and codecs not supported in macOS 10.15 are likely to become unusable when you have upgraded to that in the autumn/fall.
Apple provides detailed information with a list of supported formats, lists some of those which are being dropped, and provides full instructions for conversion. Its matching article covering iMovie gives comparable information for that app.
Among those video formats which will still be supported in 10.15 are Apple Intermediate codec, Apple ProRes, Apple ProRes RAW, AVCHD, DV, H.264, and XDCAM. Most popular still image and audio formats will also continue to be supported.
Among those which won’t be supported under macOS 10.15 are several Avid formats, Cinepak, DivX, Flash Video, FlashPix, GlueTools codecs, JPEG 2000, Motion JPEG A and B, Perian codecs (MPEG-4, DivX, and more), RealVideo, several Sorensons, and Windows Media Video (WMV) 7, 8, 9. It’s possible that some vendors may port codecs or other tools to 10.15 to support some of them in the future, although this looks unlikely at present.
Unfortunately, there’s no system-level means of checking which video, audio and still image formats remain reliant on 32-bit components such as codecs. They aren’t included in Mojave’s System Information under its Legacy Software section, which only seems to cover apps and similar bundles. Most, perhaps all, of those listed in the Components section are provided in 32-bit form and will be unavailable in macOS 10.15, but there doesn’t appear to be any listing of formats which are supported in QuickTime X.
One thorough and reliable way to identify all types of code which remain 32-bit only is my free app 32-bitCheck, available from Downloads above. Use this to scan the classic QuickTime folders of /Library/QuickTime and /System/Library/QuickTime, and you’ll see that each and every codec in them is 32-bit only, thus doomed. Here, that means the loss of a total of 42 codecs. Because QuickTime X uses a different mechanism to incorporate codecs, I can’t see any way of discovering exactly which formats will remain supported. If you know of one, please let us know.
Finally, for those who become marooned in macOS 10.15 but require access to old formats, one solution may be to run Mojave in a Virtual Machine, such as VMWare. Until the release of 10.15, it’s not clear how successful that might be, but I’m sure that VMWare will be working very hard on it.