I have a small question though: do we need the opposites too (I mean `-fno-full-guard-reasoning` and ` -Wtoo-many-guards`)? I cannot think of a scenario where we do or am I missing something?

A scenario where `-fno-full-guard-reasoning` could be useful: you add `-ffull-guard-reasoning` to ghc-options in your .cabal file, but switch it off in an OPTIONS_GHC pragma in a specific file. But `-f` and `-W` flags always come in pairs, so that is easy. You would add `-Wtoo-many-guards` to `standardWarnings` in compiler/main/DynFlags.hs. Others? If I get the green light, this is a matter of minutes to change :)

If it is not too much trouble, please submit your patch to Phabricator. Then others can review, and harbormaster can validate it.