stack ghci (aka stack repl) is the one part of Stack that uses GHC directly.
In principle†, I can see no barrier to: "if specified GHC <= 9.14 then ‘Stack do X’; otherwise, ‘Stack do Y’ ", as long as any changes (there may be none of note) to the user experience as between those two limbs are well-documented.
The ‘if specified GHC <= 9.14’ limb will have to stick around until Stack no longer supports GHC 9.14. (Currently, Stack supports GHC >= 8.4.)
† ‘In principle’, because, in practice, somebody has to be both willing and able to code the change. (If I am able, I am willing.) A ‘budget’ might increase the number of able people who are also willing.
EDIT: Corresponding issue created at Stack’s repository: