Unfortunately, (and I think this was explained when they were removed) Social Groups were an extremely time intensive "home made" add-on that we created for the site. They were cobbled together from a variety of different pre-existing add-ons that we were not the creators of; none of us are "professional Xenforo add-on creators" as it is, so this was entirely premised on the idea of using these pre-build add-ons to make SGs work.
And then . . . as happens . . . the original creators stopped supporting them. And then they broke. Because when stuff isn't supported, new versions break it. We tried as hard as we could to find a workaround, and we delayed updating the forums for literal years in an attempt to keep it around and find a way out of the predicament, but unfortunately, it wasn't possible. There was no other replacement to those add-ons, and attempting to jury-rig a brand new "Social Groups" variation from other such add-ons would have taken a ton of time and luck to even get something resembling the original result.
And all for what? So that in a few years, those new add-ons would also become outdated, and we'd have to go through Social Groups dying a second time? Unfortunately, I feel as if that was the inevitable outcome, and if we had known that from the start, we may never have had SGs at all. I personally feel that having something and then taking it away is worse than never having it at all, and trying to add it back in the same way (and only lose it again in the future) would be refusing to learn from the previous situation.
To clarify again, the original SG add-on was not a simple one-and-done installation that worked flawlessly. Loki combined a variety of different functionalities to create the final result, and key add-ons in that creation ceased to function. And that we could see (as we spent over a year delaying the removal of SGs), there was no simple replacement add-on. Its possible something like that could come to exist again, but its a risky endeavor building a major functionality of the site around add-ons that we have no control over and no ability to fix.
Just my two cents, hope it sheds some light on why this isn't likely to be feasible.