Rimworld

Depends how far back you go. Obvs, not all mods are keeping up as the game updates, but if anything, I find that the modding community has only every grown and gotten better with time.

I think that mod versioning was introduced in 1.1 (so about when Royalty released), which made it a lot easier to keep track of what works and what doesn't, so you might have issues with mods from before then, but honestly there'll almost certainly be a replacement if something was popular and was discontinued.

There are still an ungodly amount of mods for literally anything. All of the Vanilla Expanded mods are very high quality, plenty of older mods are still either updated or have been remade, and there are plenty that integrate with the new systems added by the DLC.

Mods are still going strong. Many of the big devs are still kicking out new and innovative mods to transform the gameplay.

Honestly, if anything mods have just gotten better from them. The vast majority of mods you'd recognise from then are either still updated or have been forked/continued or superseded.

There's SO MANY MODS. Many of the old ones are still kicking, some were adopted, some by the original developers. If you plan on doing a heavily moded gameplay, I'd suggest looking into optimization guides tho, some mods that used to be a must have back in the day now have better, performance-friendly options.

There's some that aren't updated but if they were big they were probably replaced. There's so many of them. You'll be fine installing mods, just check the description and comments on the Steam workshop page.