Preview
"Books and journals want (2a), not ((2)a)."
But there is no (2a) in the text, so every time the reader sees a cross-reference, he or she has to reconstruct it. (2b) is even more work, because the two elements aren't on the same line …
The (2)a you're trying for you is already easier to interpret; but I think I'd change the approach and go for something like:
2 strong past forms
2a. I painted last week.
2b. I applied last week.

Nothing has changed in this regard in Word 2016.

My guess is that people who want the format you requested are doing it manually, by typing the references.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional