They're versions of the same thing, and 9.12.1 was chronologically before 9.10.3. It looks like you can find the entire chronology at https://www.haskell.org/ghc/blog.html

OK, side note, whoever decides the GHC release numbers should be pulled aside and given a quick talk about how version numbers are supposed to work.

Anyway, looks like or-patterns just landed as a GHC extension:

    foo (Bar; Baz) = 0

I still like OCaml's (built-in) syntax better:

    let foo (Bar | Baz) = 0

Which I feel stems from its elegant decision to use the pipe character as the 'alternative pattern prefix':

    let foo = function
      | Bar
      | Baz -> 0

> whoever decides the GHC release numbers should be pulled aside and given a quick talk about how version numbers are supposed to work

How is that?

> whoever decides the GHC release numbers should be pulled aside and given a quick talk about how version numbers are supposed to work.

9.10.3 is a patch for the 9.10 version which was published after 9.12.1 but relates to an older version. Haskell keeps maintaining multiple versions of the toolchain.