RESTEasy needs to be upgraded from the 3.x releases for RESTEasy to 4.x. The 4.x branch will be the maintain branch moving forward. For WildFly this does mean some changes. Some things have been removed, some things have been moved and some things have been deprecated.
This is not an exhaustive list of what has changed, but a list of what was found during the migration from 3.x to 4.x.
The org.jboss.resteasy.resteasy-jaxrs module will be deprecated and split into 4 new modules.
The org.jboss.resteasy.resteasy-jaxrs will add these dependencies and export them. However it may be removed at some point.
The following have been removed as they now have Jakarta RESTful Web Services replacements.
@Suspend and org.jboss.resteasy.spi.AsynchronousResponse have been removed and replaced with @Suspended and javax.ws.rs.container.AsyncResponse repectively
The StringConverter is replaced by a ParamConverter.
One type, that is likely not heavily used, is org.jboss.resteasy.core.ResourceInvoker was moved to org.jboss.resteasy.spi.ResourceInvoker.
Possibly more used would be the GZIP filters moved from org.jboss.resteasy.plugins.interceptors.encoding to org.jboss.resteasy.plugins.interceptors. A workaround for this can simply be adding the resteasy.allowGzip property.