Hi Greg,
Sorry for the slow reply.
Generally speaking, we don't see a lot of problems with our own production code when moving between SE versions. This is partly because Richard Opalka does a good job of keeping on top of upcoming SE versions and sends up fixes. In general these are not big things though. A big exception was in the security area during the transition between SE 11 and SE 17, when SE removed a number of packages that our legacy Picketbox security layer used, requiring us to remove support for that (long deprecated) layer.
Note though the WildFly incorporates hundreds of components and if some of those components had to do significant work to be compatible with a newer SE we might not be aware of it.
However, that doesn't mean WildFly 32 will be able to certify as EE 10 compatible when running on SE 21. Certifying as compatible means passing all tests, and if a test cannot be passed on SE 21 because of a problem in the test itself, then we can't certify. But that's a test problem, not a WildFly problem. We currently see several dozen failures out of over 70,000 tests when running on SE 21, none of which are due to problems in WildFly.
As David Everly pointed out, EE 11 specifically calls out support for SE 21. However, an important improvement that Jakarta's done in recent EE releases (I believe since 9.1) is introducing the concept that it should be possible to certify on SE releases that come out after the EE release. The goal is to have the specification and the TCK not limit implementations like WildFly to SE versions that existed when EE was released. So, when WildFly or other communities find problems like our 40+ test failures, we can and do file issues with the relevant Jakarta spec communities to try and get tests corrected so they work on SE 21, or to get them excluded from the TCK. (Jakarta has formal processes for this kind of thing.) There are issues like this in progress, and if they get resolved before WildFly moves on to EE 11 support then we'll aim to certify as EE 10 compatible on SE 21.
Best regards,
Brian