The official Homestuck site will still let the original SWF files load as long as you have something that can play them. Ruffle works fine. You can also use one of the few browsers that still supports PPAPI plugins (like Falkon) with the official Flash plugin.
But personally I’d say to just use The Unofficial Homestuck Collection, which is more pleasant to read through than the original ever was.
I dunno, I expect the Deck to last far longer than the average console if anything. It’s a PC, so the games are pretty much guaranteed to keep coming for decades to come, as they have for decades past.
The hardware will fall behind, so I think the point where the newest Triple A games won’t be playable will come within a few years, but I bet whatever visual novels or pixelated indie games release in 2035 will still run just fine on it.
Plus, it’s designed to be repairable, unlike most consoles. And even if Valve stops maintaining SteamOS for the Steam Deck, you’ll still be able to install other distros, so software support isn’t something I’m very concerned about either.