Java 13 was never supported by Tycho, because it has never been necessary so far.
Tycho 1.7.0 was trying to be more tolerant and was resolving against a different EE without warning much; while it worked, this was a confusing state both for usage and maintenance of Tycho. But with Tycho 2.0, EE management becomes more strict (and more clear/simple), so it now uses JavaSE-13 to try to resolve dependencies if running with Java 13, and since JavaSE-13 isn't supported by Tycho, it just fails with an explicit message.
I suggest you just use Java 14. If you really need support for Java 13 in Tycho, a contribution to add the .profile file and some tests to verify it works would be welcome.