There is no real changelog, but you can look at the following things:
The release changelog, which should contain most user-visible changes, including new features and bug fixes:
The git log, which is the "real" changelog
The file https://github.com/mpv-player/mpv/blob/master/DOCS/interface-changes.rst documents changes to the command and user interface, such as options and properties.
C API changes are listed in https://github.com/mpv-player/mpv/blob/master/DOCS/client-api-changes.rst
The file
mplayer-changes.rstin theDOCSsub directory on the git repository, which used to be in place of this section. It documents some changes that happened since mplayer2 forked off MPlayer. (Not updated anymore.)