While having a package system would be certainly desirable, I wouldn’t make the perfect the enemy of the good. A repo of functions is an improvement over current state and could potentially pave way for packages while being substantially lower-effort. I am also not sure we actually have a good idea what kind of functions people would like to reuse and share…
Stan can compile standalone functions files since 2.16 (unless the code broke recently) Standalone compilation of functions block · Issue #2267 · stan-dev/stan · GitHub althoguh I don’t think the feature has been used much and I am not sure the feature get eventually fully integrated in the interfaces though (e.g. Integrate standalone function compilation with RStan · Issue #422 · stan-dev/rstan · GitHub is still open).
For some even broader context: There was a discussion of “modules” (which is more than a functions library, really more of a reusable library of submodels) here: Stan++/Stan3 Preliminary Design (AFAIK the discussion didn’t really end with a conclusion and I remember not being a particularly helpful element of the discussion myself)