@grainloom there's the RNRS (where N is a number between 5 and 7, inclusive) standards, anything coded to those should be perfectly portable, though I've never bothered checking.
Semi-related, racket is being reworked to run on top of other scheme implementations (chez scheme in particular, with other runtimes being an option in the future) so that suggest high portability
@grainloom Schemes vary, with RnRS version and anything calling the OS.
But see SLIB which runs (ran?) on many schemes: "Bigloo, Chez, ELK 3.0, Gambit 4.0, Gauche, Guile, JScheme, Kawa, Larceny, MacScheme, MIT/GNU Scheme, Pocket Scheme, RScheme, scheme->C, Scheme48, SCM, SCM Mac, scsh, sisc, Stk, T3.1, umb-scheme, and VSCM. " https://people.csail.mit.edu/jaffer/SLIB
R7RS also leads to portable code: http://snow-fort.org/pkg. I have some libraries running on all of Chibi, Gauche, Kawa, Larceny and Sagittarius.
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