Re: dropping some OCaml versions from OPAM CI, I think it’s a good idea to save on electricity and to make CI reports shorter and easier to process.
Concerning which OCaml versions to support, Debian is currently at 4.13 (oldstable), 5.3 (stable), and 5.4 (testing). RHEL/CentOS 8 is at 4.07 (too old!), 9 at 4.11, and 10 at 5.2. So, maybe you could add 4.11 to your list of significant releases, as a gesture towards old Linux distributions.
Concerning compiler variants, I agree with reducing the number of combinations. I think Flambda and FP support are largely orthogonal, so testing Flambda + FP may not be useful.