Tony StillApple has published a list of media formats (mostly video) that macOS will no longer support after Mojave. Part of the removal of support is the demise of the old QuickTime 7 media player (that some of us also bought a Pro licence for many years ago): QT7 is much more capable than the current QT Player and provides some of the old formats (codecs).
Full lists of the formats that will be supported and not supported are on an Apple Support page. If you have important elderly media files then now is the time to convert them to some more modern format; just make sure that you minimise any loss of quality by careful choice of the new format.
I'm personally sorry to see the demise of JPEG 2000, an improved version of JPEG that I don't think anyone ever used.