Bit of a surprising comment - Xen is very widely used - even all of AWS is Xen (I believe), are you perhaps thinking of XenServer (Citrix's OS / Distro based on Xen)?

If you do mean XenServer, well - yes some large places are still using it but it's definitely dying out after they proved very slow to deliver new features then put a restriction on free / open source users as of 7.3 onwards that prevents clusters great that 3 hosts rendering mostly useless.

XCP-ng (https://xcp-ng.org/) is a fork of XenServer which is picked up where XenServer left off and is doing a lot in a short period of time (IMO) to add new features and build / support the community.