WildFly currently adds default dependency for org.jboss.xts module when deployment defines some of the compensations annotation. Provided by XTSDependenciesDeploymentProcessor see https://github.com/wildfly/wildfly/blob/master/xts/src/main/java/org/jboss/as/xts/XTSDependenciesDeploymentProcessor.java#L58 (in fact XTS dependency is needed only when compensating framework works in distributed manner but it's absolutely ok by my point of view to add the dependency as it's currently done).
But if deployment uses just XTS interfaces and defines WS handlers in descriptor then user is forced to apply the dependency on its own.
E.g. eap quickstarts does so
It implements Durable2PCParticipant and the XTS transaction context processing is done by specifying the xts handler in descriptor (https://github.com/wildfly/quickstart/blob/11.x/wsat-simple/src/main/webapp/WEB-INF/classes/context-handlers.xml#L24).
It would be good to add implicit dependency in such cases for user does not need to provide it on his own.