In principle, reactive and callback-style libraries could eventually use Virtual Threads as backend as well, but I expect for them to fall out of fashion. After

In principle, reactive and callback-style libraries could eventually use Virtual Threads as backend as well, but I expect for them to fall out of fashion. After all, they were introduced because the existing threading support in Java did not adequately support massively-concurrent applications. There will surely be new frameworks that will take advantage of Virtual Threads and will eventually require to run on the first Java LTS version with Virtual Threads support. Other projects might offer Virtual Threads as a backend, maybe even by default, but will continue to support traditional backends for the foreseeable future, e.g., Apache Tomcat.