actually, after reading the relevant section on Wikipedia, I found Michael didn’t leave the group until quite a bit later, 1984 or 1985. they even released a few albums as “The Jacksons” still with Michael, even as he released his first solo album or two.
now as said, many other databases have them as separate entities, and after going through the Spotify releases and playlists for both The Jackson 5 and The Jacksons, neither of them has the other as a track artist on their releases (i.e. an album by The Jacksons with a track credited to The Jackson 5, or vice versa). I also didn’t notice any Jacksons tracks credited to The Jackson 5, but I only know like, 3 of their songs, so there’s a real good chance I missed all of them. 
almost all of The Jackson 5’s releases are copyrighted by Motown or their parent UMG, and there have been a several rarities/compilation albums released under the Jackson 5 name since 1976.
that said, it could just be that the labels are being lazy with Spotify artist credits (wouldn’t be too surprised), and all that would be moot.
I haven’t looked at any of their concerts yet to see if they’re performing J5 songs after 1976…
the big difference between most examples of an artist changing names for legal reasons, is most of the time it’s either just in certain countries, (Yazoo, PUFFY, and quite likely the previously mentioned Suede, The Beat and others) or it’s before the artist got super popular (The Supertones/The O.C. Supertones, possibly X JAPAN, and certainly others). however in this case, the older artist name is the more popular name.
all of that to say, if we decide not to split, I do think the entity should use their current/most recent name, with the other in the disambiguation. so they’d be “The Jacksons” with a disambiguation of (known as The Jackson 5 from 1966–1976) or something along those lines… split or not, I’m more than willing to put in the edits once more discussion has happened~