So, it appears that the FHIR Java ecosystem is rapidly evolving to require Java 11 as a baseline. This is already the case for HAPI FHIR 6.x and it seems like the core data types library is also migrating to require a minimum version of Java 11. This seems to push us in a place where future versions of the FHIR2 module are going to need to depend on Java 11, and hence only be compatible with Platform 2.4+.
There are, as far as I’m aware, no plans to make this change immediately, but likely at some point over the coming year, we will move the active development of the FHIR2 module to require both Java 11 and platform 2.4+ and leave a legacy branch for bugfixes for implementations running on earlier versions.