Is Skyrim still only playable via Steam ??

I have the original release disks but removed Steam from my computers a good while ago. And although I've looked around, I can't find any other version that might be "legal".

All "legal" sources for Skyrim require Steam.
Online game sites that sell it, like greenmangaming and humble, use Steam.
The disc version also uses Steam.

Earlier Elder Scrolls games like Morrowind and Oblivion eventually appeared at GOG, so maybe Skyrim will show up there someday.
Maybe on its 10 year anniversary?
Maybe after Elder Scrolls VI is released?
Who knows?

Yes. When I bought the game, I, like many others, wasn't aware that the disk version also required Steam. Never having experienced that circumstance with a "disk" game before

Well, maybe in another few years it might appear at GOG - although I very much doubt it