Hi,
Threads subsystem has not been directly used since 7.1.0.CR1.
see
https://issues.jboss.org/browse/AS7-390 for more details.
The only usage we have is that EJB3 & JCA subsystems use some
common code from threads subsystem (parser & few RD) but beyond that
subsystem is unused aka we could remove ThreadExtension and everything
would still work.
And ever since there was no need for thread subsystem enabled/deployed in configuration but we still had it in all our default shipped configs.
I would like to remove it from our default shipped configurations as it does not effect any functionality.
Also I have introduced option to "deprecate" extension, so it now logs warning when such extension is registered (aka is found in configuration)
As goes for compatibility issues in mixed-domain this change does not effect 7.1.0+/EAP6 as they can properly operate without this subsystem.