We still have many, many customers still running on Java SE 8. If
app servers move to Java SE 11 as the minimum, you may be alienating a
huge set of potential customers.
not saying that app servers can not support Java SE 11. I'm just
indicating that providing the minimum version as Java SE 8 is more welcoming.
There's advantages and disadvantages. Not having a guarantee that it works on Java SE 11 might well alienate another group of potential customers and make them decide to do Greenfield development using another technology that does support JDK 11.
Keeping an ear to the ground a little, developers are quite eager to start Greenfield with the latest JDK (latest technologies/latest versions of them in general). Having to start new development on a ~6 year old version of Java is, well, not overly exciting.
What about taking Carlos' suggestion one step further and introducing individual Jakarta certifications for JDK11 and JDK8? A vendor may claim "some degree" of compatibility with JDK11, but the proof's in the pudding.
E.g. "Jakarta EE 9 for JDK 8 certified" and "Jakarta EE 9 for JDK 11 certified"