Since Python is used for webdevelopment, it would be very beneficial for developers to have subtitle files as default mimetypes.

What standard specifies these MIME types?

text/vtt is has IANA registration 
https://www.iana.org/assignments/media-types/text/vtt 
and .srt is plain/text https://www.matroska.org/technical/subtitles.html#srt-subtitles

Those two looks legit to me.

New changeset d74da9e140441135a4eddaef9a37f00f32579038 by Josephine-Marie in branch 'main':
bpo-45411: Update mimetypes.py (GH-28792)
https://github.com/python/cpython/commit/d74da9e140441135a4eddaef9a37f00f32579038

Additions to mimetypes have been backported in the past, with the thought that these are not new behaviour added but only entries in a registry (and bringing the python module in line with external data sources) so there’s value and no risk in releasing these additions in point releases.

I don’t have a link to the discussion about that, and looking at 4 or 5 recent PRs to mimetype I see that backports haven’t been consistent.  So maybe another ticket would be to synchronize the dicts in active branches + record the backport policy somewhere (not sure a comment would be enough, easy to miss at the top of a long dict).