Equinox no longer ships execution environment definitions for modular JVMs and rightfully so as the fact that it's Java 13 doesn't ensure all modules that make the full Java 13 are included.
To workaround that Tycho itself ships such definitions (hoping that people don't run tycho with stripped down JVMs).
Today support for Java 14 was added but support comes at a price and it's just not feasible to keep adding more and more not supported JVMs.
Thus proposal is for tycho to ship and support execution environments that are only LTS Java, latest Java version and in development Java version, aka 11, 13 and 14. This would mean dropping Java 10 and 12 right now.
Red Hat Eclipse Team