I’m surprised to hear this from an avid Haskeller and I think it might give the wrong impression to those who are less familiar with Haskell. I’m sure you know this, but for the benefit of others, projects don’t have their own extensions, they just may or may not use some of the extensions provided by GHC. Anyway, that practice is now diminishing given the GHC2021 and GHC2024 “standards”, which just enable a fixed set of stable extensions.

And regarding using specific compiler flags, well, projects almost never do that.