The old "API" that signaled rotation as a metadata value has been replaced by DISPLAYMATRIX side data quite a while ago.
There is no reason to make muxers/demuxers/API users support both. In addition, the metadata API is dangerous, as user tags could "leak" into it, creating unintended features or bugs.
ffmpeg CLI has to be updated to use the new API. In particular, we must not allow to leak the "rotate" tag into the muxer. Some muxers will catch this properly (like mov), but others (like mkv) can add it as generic tag. Note applications, which use libavformat and assume the old rotate API, will interpret such "rotate" user tags as rotate metadata (which it is not), and incorrectly rotate the video.
The ffmpeg/ffplay tools drop the use of the old API for muxing and demuxing, as all muxers/demuxers support the new API. This will mean that the tools will not mistakenly interpret per-track "rotate" user tags as rotate metadata. It will _not_ be treated as regression.
Unfortunately, hacks have been added, that allow the user to override rotation by setting metadata explicitly, e.g. via
See references to trac #4560. fate-filter-meta-4560-rotate0 tests this. It's easier to adjust the hack for supporting it than arguing for its removal, so ffmpeg CLI now explicitly catches this case, and essentially replaces the "rotate" value with a display matrix side data. (It would be easier for both user and implementation to create an explicit option for rotation.)
When the code under FF_API_OLD_ROTATE_API is disabled, one FATE reference file has to be updated (because "rotate" is not exported anymore).
ddef3d9 avformat, ffmpeg: deprecate old rotation API
cmdutils.c | 10 +---------
doc/APIchanges | 8 ++++++++
ffmpeg.c | 41 ++++++++++++++++++++++++++++++++++++-----
ffmpeg.h | 1 +
ffmpeg_opt.c | 12 ++++++++----
libavformat/mov.c | 2 ++
libavformat/movenc.c | 4 ++++
libavformat/version.h | 5 ++++-
8 files changed, 64 insertions(+), 19 deletions(-)