Given that WildFly now mandates JDK 11 and Narayana also mandates JDK 11 then the jboss-transaction-spi project could also mandate this.

You can see the Narayana mandate here:
https://github.com/jbosstm/narayana/blob/main/pom.xml#L556

You can see how the WildFly is doing it by following information in the pom's (here abridged):
https://github.com/wildfly/wildfly/blob/main/pom.xml#L29 ->
https://github.com/jboss/jboss-parent-pom/blob/jboss-parent-39/pom.xml#L162 ->
https://github.com/jboss/jboss-parent-pom/blob/jboss-parent-39/pom.xml#L720 ->
https://github.com/wildfly/wildfly/blob/main/pom.xml#L165

Given the jboss-transaction-spi uses the same org.jboss:jboss-parent as WildFly providing a maven.compiler.source of 11 would hopefully be OK. It might also be appropiate to set the maven.compiler.target to 11 too (https://github.com/jboss/jboss-parent-pom/blob/jboss-parent-39/pom.xml#LL322C25-L322C55)