Watching YouTube on Steam Deck / SteamOS? You need to grab VacuumTube which makes the whole experience a lot nicer. This is not a custom client, instead it wraps Google's own "YouTube Leanback" that's used for consoles and smart TVs in Electron and pretends to be the official YouTube app (as Google block it on PCs).
Quite a clever solution, although one Google is probably not particularly happy to see. Especially since it has a built-in adblocker too. The result is that for Steam Deck, SteamOS and any other media centre PC you just get an overall better experience when compared with running YouTube via the web browser.
It's available on Flathub, so you can install it in Desktop Mode via the Discover store. See my guide on installing extra software.
When you've added it to Steam and launch it in Gaming Mode, it feels like it was made for the Steam Deck / SteamOS systems. Controller support out of the box too, making it the perfect YouTube companion for you.
It just goes to show how easy it would actually be for Google to do directly. Just like how we have an official GeForce NOW app for the Steam Deck too.
The code is on GitHub, it's open source.
Hope you find it useful! I certainly do.
Will give it a shot.
No sponsorblock? Not touching youtube without it. The best way to use youtube on steam deck remains a kiosk mode youtube webpage with all the modern and configurable adblocks and sponsorblock and a gamepad preset from community templates
Since it appears to be a TV client, I suspect you could run https://github.com/dmunozv04/iSponsorBlockTV to get sponsor-skipping. Not the most elegant solution, but possible.