Mmm, well, there's a logic to banning it in older formats. The idea is that Lurrus (and maybe Zirda too) have restrictions that basically are not restrictions in old formats. The "cost" of playing with an eighth card in your hand is supposed to be a disruptive restriction on your deck. In Standard, probably Pioneer too, these restrictions are probably very real. In Legacy and Vintage they're really not.
I think that argument falls apart in Modern though. Zirda is sorta a non factor if only because Legacy and Vintage have the tools to go infinite with her and modern does not, but Lurrus is generically good in those formats as well and there are already lists that show the deck building cost can be overcome.
The longer modern is a format, the more and more true it becomes. I doubt that the format will ever be a Lurrus format in the long run, but I can easily imagine it being a companion format forever. I play a living end deck in modern and without having to do a single thing my deck was already good to run Zirda. And while her effect is minimal in the list, it is still a free 3/3 creature if I need it in a grindy match.
Likewise a lot of burn decks started running Obosh, the Preypiercer with minimal changes to their lists, Gyruda, Doom of Depths is very likely just going to be a deck on its own with spark double and the like just reanimating 10 drops, and my understanding is that Kaheera, the Orphanguard is something that a great many creature-less decks are running.