As a lot of users pointed it out, they're not the first to do a reunion. However, I think the scale of their activities and the fact that at the time the three that weren't signed with SM could just perform at SMTown and do their own mini-concert is notable.
I wonder what kind of contract they signed that divided the profits for their activities between SM and the other agencies the girls were with.
Certainly the experience they gained from it will be important on their next reunion where I'm really hoping that they go on tour (they should've done it in 2022, but their schedules made it impossible unfortunately)