For example, the Seinfeld set was still in storage and reused when they did the Curb episode.

The Home Improvement Tool Time set is in Tim Allen's storage garage.

Does anyone know if any '90s (or even before that) sitcom sets still exist? If so where can I see pictures?

Here's a photo of the original set usedc on I Love Lucy. This was when they went to Hollywood

I believe that the Cheer's set is at the Smithsonian Museum; I also believe this is where parts of the All In The Family Set is.

A long time ago I did a commercial for a board game based on The Honeymooners and they brought in the original backdrop from storage. It was a very nondescript tan/brown. I imagine it still exists although i have no idea where it is. They didn't have the original furniture but it was recreated. After we finished shooting for the day I went to the prop room, got a golf club, went to the appointed spot and said, "Helloooo ball......".