Seems fine to me. I haven't tested at all, just reviewed the change on Phab. Everything here makes as much sense as I can make out of it (take that for what you will :-)

Looking at what should be in the legacy provider, I fell down this rabbit hole by looking at a completely stock build of 3.5.4-dev on my Mac (I should probably check on a FreeBSD system and see if it is any different, but that would require a small amount of effort and I'm about to go to a dinner) and see this list as the primary objects that end up in the legacy provider:

How did you come up with these changes? What was the methodology that got this changeset?

I’m not entirely sure what kind of approval from secteam is being sought. If someone in core would like to help me understand what kind of review is expected, I’d be happy to undertake it.