Roy Thomas left the first time in 1980 (Shooter related). I don't know exactly when he returned to Marvel. That I can recall, he did a 12-issue SAGA OF THE SUB-MARINER series with Rich Buckler in 1988-1989.

And then the Sub-Mariner 70th anniversary issue discussed earlier in the topic. So that shows Thomas at least doing sporadic work for Marvel as recently as 10 years ago.

For some reason, my local comics dealer said the older comics writers and artists are not offered work by the big two publishers and have trouble getting work, and they kind of struggle. Which is really awful, because guys like Wein, Wolfman, Thomas, O'Neil and Moench are the ones who largely built the industry in the 1970's and 1980's as the continuity and characters largely remain. Even the editorial leadership at Marvel might not have jobs, if not for Roy Thomas.