My takeaway is that as of EntireX 10.3, both the server and the RPC client will need to be on Java 8. However, as of EntireX 10.5, the server will continue to remain on Java 8, but the RPC client will be compatible with Java 11.
Thank you for the detailed explanations. If my takeaway is correct, you have provided exactly what I was looking for.
While not the same product, one of the things that had me wondering about the Java 11 support was that it was mentioned in the ApplinX 10.4 details on page 6 of the Mainframe Integration 5 Years Roadmap (March 2019 edition).
http://techcommunity.softwareag.com/download/webMethods/ApplinX/Mainframe_Integration_Roadmap.pdf