Also, there would be a conflict with #2763637: D7 taxonomy term fields are no...

Also, there would be a conflict with #2763637: D7 taxonomy term fields are not migrated with allowed vocabularies for which I "borrowed" some code from here. That is much simpler though. If it gets in first, this would need a reroll.

It seems we have a lot of issues where we could reduce common code in patches if we plan common issues to handle the basic foundation first (like making field settings available to instance migration in this case). It is difficult to determine that, however, but we could try.