UO Second Age

I specifically remember on OSI that you could not continue to be in the thieves guild if you dropped stealing below the required 60 skill. This also meant that you could not use disguise kits.

I had a dexer thief, which I later turned into a pure dexer and when I dropped stealing skill I remember getting kicked out of the thieves guild.

Evidence?

Not sure which era you are talking about but just after trammel came in I had an assassin - got to 60 steal to join the thieves guild, joined, and then dropped it to 0 to build my combat skills back up. Never got kicked out and it worked just fine.

You will however get kicked out if you get a murder count - maybe you were reported around the time you dropped your skill?