Is this of any value? Hopefully.
As I’m writing this, I haven’t played under the new rule yet. In case you are wondering, the new rule is that instead of just casting the Companion from outside of the game, you have to pay three generic on your main phase with the stack empty. This is going to make at least some of the Companions completely unplayable.
First, there’s a huge difference between doubling the manacost (like Lurrus and Kaheera) and adding 60% to it (like Yorion). This doesn’t necessarily affect each Companion the same, but it seems to me that Yorion, which is/was the most played Companion in Standard, is going to take a much lesser hit than any of the others (well, Keruga took a hit for other reasons and would have probably been way too powerful, if the Fires hadn’t been banned as well).
Second, you can’t count on these to smooth out your curve. Without the new tax, you could could plan out your games in a certain way (which is bad from the point of game play, but good from the point of view of deck design). If you always have access to a card of certain cmc, you know that you can play it on a specific turn (if you have the mana, but you should design your deck to have it). This also means that you can put more cards into the other slots on the curve, which in turn makes it more likely for you to be able to curve out. Gyruda, Obosh and Umori are the big losers here. With Obosh, if you can’t flood the board and then curve out with it, limiting your deck to odd-drops is just not worth it. With Umori, paying extra three to possibly get a discount for future creatures seems unlikely to be enough of a benefit (unless you are playing a very specific deck, which is willing to skip a turn). Gyruda… just no. I don’t think you can play it anymore.
Third, certain signalling is not going to be worth it anymore. Take Umori, for example. In many cases, you don’t want to tell your opponents that you lack in interaction? Sure, we have plenty of cards in Standard with various interactive abilities, but this is still a strong signal that your black deck will probably have less removal than usual. Neither will your Obosh deck have [card]Embercleave[/card] nor will your Lurrus decks have [card]Murderous Rider[/card] or [card]Leyline of the Void[/card]. Obviously, this cost already existed, but making your sacrifices in deckbuilding obvious to your opponents is something you have to take into account even more now.
All in all, I’m a little bit afraid that Yorion pretty much survived unscathed, while the others are unplayable. This might very well mean that Yorion is going to continue to dominate in the near future and might require a ban. The others will still see occasional play, but will mostly not be worth it. In some cases, the sacrifice is minimal (like Lurrus in the Cycling decks), so it will still be in some lists, but as the deck usually already uses all of it’s mana each turn, it won’t actually do much.
We will also still probably see certain Companions in places where you don’t won’t to see them, like Kaheera might as well still appear as part of certain Control decks, which don’t have any creatures anyhow, so it’s basically free. This perversion of the mechanic might prompt another change to the rule.
All-in-all, this is just a mess. If they wanted to still have Companions be relevant, in my opinion they only managed to keep one of them as such. I think the various rules proposed by a number of people, where using a Companion would cost you a card in some way, would have been better, but I do assume they tested that. Not sure why they decided not to go the way they did. Also, my trust in their testing is not very strong at this time (and I’m pretty sure I’m not the only one).
Final note: I haven’t checked this, but apparently WotC thought that losing a sideboard slot was big enough cost to make people not want to use Companions. This seems ridiculous. Sure, that’s a cost, but compared to the benefits of having one, it’s miniscule. In some cases, the Companion can even perform the role of a sideboard card anyhow. Lurrus is a pretty good play after a sweeper.