Macworld:

Apple sheds the past as necessary to provide new features in its products. With macOS 10.15 Catalina, Apple gave up on 32-bit software after two years of Apple warning about that event coming. That meant iPhoto and a host of third-party apps could no longer be used after upgrading. But less remarked upon were video formats that relied on software components Apple had never updated to 64-bit compatibility.

There’s a long list of formats dropped in last fall’s macOS update, and it includes faves from years past: RealVideo, DivX, Sorenson 3, FlashPix, and many great hits that eventually were superseded by improved encoders.

This is one of those issues that, even though Apple warned users for several years ahead of this transition, still might catch less tech-savvy customers out.