The documentation you quoted does read to me as compatible? The database it is referring to is the one hardcoded in the module – not the one assembled from that and the host OS. But, maybe this is just the vagaries of language and perspective at play.

Anyway I do agree it is an unexpected behavior change from the perspective of a user of the MimeTypes class directly. To get the best context for this change, it's useful to run through the long history of the issue that drove it:

https://bugs.python.org/issue4963

Note, that discussion never touched on the use case of instantiating a MimeTypes class directly and there are apparently no test cases covering this particular scenario either. With no awareness of this perspective/use case it didn't get directly addressed.

Perhaps all MimeTypes instances should auto-load system files unless a new __init__ param selects for this new "clean" behavior?