A lot of our patches get applied to either 4.x (previously 8.x-3.x) or 7.x-3.x, but are never ported to the other, and I suspect the two are out of sync in the way they work.
Example: ✨ Handle case of only email before @ returned. And match email via user_load_by_mail, not custom SQL. Downport adjusts the way the user’s email address is matched against the accounts in the database, but it is still very different from the way 4.x does the matching.
I suspect no one is trying to do an upgrade in place to get from core 7.x to 9.x or 10.x, so this may not matter that much, but also I wonder if we have lost anything important since the branches diverged.
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I can tell you that when the 8.x-3.x branch was created, that was te last time that there was any parity between the 7.x branch and the later branches.
I know there are features I added to the 8.x-3.x branch that I didn't bother backporting to 7.x-3.x.