Apache SIS is an implementation of GeoAPI interfaces, which is a project from the Open Geospatial Consortium (OGC). The current GeoAPI version is 3.0.2. The policy on GeoAPI versions is to be decided by OGC, but current roadmap envisions two next versions: 3.1 and 4.0.
GeoAPI 3.1 would be a version fully backward compatible with GeoAPI 3.0.2. If there is a demand for GeoAPI 3.2, 3.3, etc., all of them would be backward compatible.
GeoAPI 4.0 would have some compatibility breaks with GeoAPI 3.x line when it allows better leveraging of new standards. GeoAPI 4.1, 4.2, etc. would all be backward compatible with GeoAPI 4.0.
GeoAPI 5.0 may be created if some standards change in incompatible way after GeoAPI 4.0 release (not currently planed).
Apache version lines follow the two GeoAPI lines as below:
Even after SIS 2.0 is released, we plan to continue to maintain the SIS 1.x line for a few years. It would give some time to users for migrating from GeoAPI 3.x to GeoAPI 4.x at their convenience. Actually we are already doing that process for many years. The SIS "geoapi-4.0" branch is doing development on top of GeoAPI 4.0 development branch, then the work is ported to master (which depends on GeoAPI 3.0.1) from times to times. Currently only the SIS versions on top of GeoAPI 3.0.1 make the official SIS releases.
Apache SIS 1.4 has been released in October 2023. Next version will be 1.5. Work planed for 1.5 release includes: