The initial set of OpenGIS functions were implemented in
CALCITE-1968 but there are quite a few more to implement.
Each function can be implemented separately, and with very little effort: you typically need to find the commented function in in spatial.iq (it will be followed by the comment # Not implemented), add one or two queries as test case, add a method to GeoFunctions.java, iterate until the test cases work, then add a row to one of the tables in reference.md to document that the function is now implemented.
I suggest that each pull request implements a few related functions. A good place to start would be ST_X(geom); ST_Z(geom) is very similar and is already implemented.