Bumping. Would be deeply interested too.

I did some search and didn't find any recent statement on that.

There are some that run on generic R[67]RS, but many libraries tend to target a specific implementation (chicken with its eggs, all the guile-whatever libraries). You can check out Akku (akkuscm.org) or Snow for Scheme-oriented package managers.

It will depend on which implementation specific behavior a given library uses. For example, guile has `#{}#` syntax for verbatim atoms. If a library uses that it will not work on any other scheme. Guile and gambit also have divergent behavior when e.g. parsing char literals `#\)a` parses correctly in guile but not gambit, and on the other hand `'a'b'c` reads as `c` in gambit but `a'b'c` in guile. And these are just issues with variant behavior in the reader.

The list of these kinds of edge cases is long so it probably winds up being easier to try to just run a library to see if it works. I doubt anyone has done a systematic survey.

There are often libraries specific to the Scheme one uses, either built-in or external but easy to install. When writing code one has the choice of picking the one that is specific for the Scheme dialect, or trying to get by by using SRFIs (Scheme Request For Implementation). When using SRFIs, there is a good chance, that they are also available in another Scheme. Perhaps one has to change how things are imported a little. At least for popular SRFIs. However, often (maybe this is a requirement for a SRFI? idk) there will also be an example implementation given with the SRFI, which might allow for easy porting to the Scheme one uses.

I once ported a library I wrote in Racket from Racket to GNU Guile. Since I made use of Racket's for loop forms, I had to rewrite those parts using different constructs like named let in GNU Guile. That was most of the work. I also had to changed how it makes use of multiple cores (places -> futures), but that is to be expected, I guess.

As others have stated, it depends on the code you are porting. If you do not have many dependencies and made a choice to use mostly standard Scheme constructs, you will have not much of a problem to port it to another Scheme.