DOCS/man/changes.rst
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.rst in the DOCS sub 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.)