(I'm speaking as a player here, not as a dev/mod/representative of Lichess. This opinion is my own and not necessarily that of other Lichess team members.)
While I don't play with the keyboard extension myself, I don't think it makes much sense to disallow it.
I consider it to be an input method. There are a lot of ways you can input moves on Lichess: mouse by drag-and-drop, mouse by clicking source and target, touch screen, SAN keyboard input (
lichess.org/account/preferences/game-behavior "Input moves with the keyboard"), and Blind Mode. Of course, some of these are objectively better for ultra/hyper than others, yet for others it's debatable: some prefer touch screen, others prefer mouse.
What all of those input methods have in common, though, is that they provide a way for you to specify the source square and the destination square. The keyboard extension is no different in that regard. So if the keyboard extension gets banned, on what grounds would it get banned? Conceptually it's no different from what Lichess already has.
The only difference is that the other input methods are native by Lichess, and the keyboard extension is not. Does that mean that Lichess should ban all input methods that are not native to Lichess? I don't believe so, because that would only allow the input methods that the developers want to maintain (so do they get all the power?), and then the discussion will only shift to what the developers *should* maintain, with one side arguing that the keyboard extension should become native, and the other side arguing against (or, in case it would ever become native, arguing for its removal). So that does not resolve anything.
Besides, being able to enhance software to your liking is a beautiful thing, and fits well into Lichess' free software philosophy. As long as the enhancements do not break our Terms of Service, of course. But the keyboard extension does not: it's an *input method*, not *assistance* - you are still the person specifying what pieces to move.
I often hear people saying that the keyboard extension poses an unfair advantage, but I do not see how. In fact, there are many advantages one can have when playing ultra/hyper: being good at slow chess is an advantage, being young/healthy/having good reflexes is an advantage, having a stable network connection is an advantage, you get the idea. This is entirely unlike slow chess, where all you need to win is be good at chess, so ultrabullet has always been about more than that. Many of those are advantages more unfair than the keyboard extension: you can't just get younger, or trivially fix your connection if it's unstable. The keyboard extension, on the other hand, is available to install for everyone.