In April, Zvi Mowshowitz wrote a comprehensive guide to Ikoria. In it, he discussed the companion mechanic in what, at the time, felt like hyperbole:
Another problem is whether or not to continuously play the game of "but what is the companion going to be?" It's not like decks can simply not play one.
At that point the companion restrictions felt somewhat real, at least for me and the folks I talk to about Magic. Some were more obvious than others; Lurrus of the Dream-Den and Zirda, the Dawnwaker were clearly going to have an immediate impact on eternal formats, and indeed they did. The rest were fuzzier to me, which is perhaps a roundabout way of saying I'm not very good at Magic. Gyruda, Doom of Depths was powerful, but there's no way a mana curve starting at two would be workable. Yorion, Sky Nomad felt like a pipe-dream to set up. Et cetera, et cetera, et cetera.
Of course, in the end, Zvi was right. It all seems very obvious in hindsight.
The only thing keeping the companion errata from being the clunkiest errata ever issued is that Time Vault of Flash, many of Illusionary Mask bizarre and Waylay allow Abeyance. So companion has that going for it, which is nice. Even so, stuff like Jegantha, the Wellspring, Kaheera, the Orphanguard, Yorion, Sky Nomad, and even Lurrus of the Dream-Den will be "free" enough, at least in Standard, to mitigate the power-level errata companions got this morning (barring more companion errata, Lurrus of the Dream-Den will certainly be banned in Modern eventually). Companions won't be so ubiquitous anymore, but Kaheera, the Orphanguard and more likely Yorion, Sky Nomad control decks will still own a healthy portion of the metagame share.
Because of how "free" they are relative to their contemporaries, Yorion, Sky Nomad and Lurrus of the Dream-Den will probably be banned in Standard. A Yorion, Sky Nomad ban is virtually guaranteed at some point. Players quickly learned that an 80-card maindeck isn't really much of a drawback at all when the upside's there. In this case, it's an 8th card in hand, but running ~80 has been fine for a long time. As the story goes, while it was in development, Arc-Slogger dealt three damage per activation instead of two. So during playtesting Patrick Chapin built an 84-card deck for the sole purpose of being able to safely activate Arc-Slogger seven times. Now Arc-Slogger deals two damage and anyone who can recall a decades-old FFL anecdote knows that the drawbacks presented by playing 80 cards are mitigated by drastic and slight upsides alike.
With Fires of Invention and Agent of Treachery added to the mix, there are currently six cards banned in Standard. If you're the type of person who cares (and I very much am), that's three cards from M20—Field of the Dead, Veil of Summer, and now Agent of Treachery—and three cards from Throne of Eldraine—Once Upon a Time, Oko, Thief of Crowns, and now Fires of Invention. Six cards sounds like a lot, but as recently as 2018, Standard had seven cards banned (Smuggler's Copter, Aetherworks Marvel, Felidar Guardian, Attune with Aether, Rogue Refiner, Ramunap Ruins, and Rampaging Ferocidon), five of which hailed from Kaladesh block. So there's at least a recent precedent for this sort of thing.
People will likely kick off post-companion Standard from one of these spots:
From there, someone will figure out the right Yorion, Sky Nomad control configuration, it'll be everywhere, and that will be that. Perhaps the Bant deck will be the one to put Yorion, Sky Nomad to best use, but 80 cards does feel like a real drawback to a deck that wants its Growth Spirals so badly. When that Yorion, Sky Nomad deck happens, Teferi, Time Raveler and Yorion, Sky Nomad will likely catch bans, at which point Wilderness Reclamation'll get tagged with an obligatory ban too, lest it run rampant without Teferi, Time Raveler around to keep it in check. None of this sounds great!
Then again, I've been wrong before. Fingers crossed.