Deprecate IM image format

[radarhere]

#9101 (comment)

we could consider deprecating some of the obscure core plugins. There are definitely a few added during the early PIL days, where it was really hard to find sample images when adding tests decades later! These should be long deprecations, which could be dropped if we get feedback of real use.

When looking for specification for the IM format for #9800, I was unable to find anything unrelated to Pillow. I suggest deprecating with a two year deprecation period.