AFAIK the effort is still ongoing. I believe LWN had some nice coverage on that, with the last majir pain point being attaching a security context to sockets in a way that suits all interested LSMs.

Anyways, I would not expect to have LSM stacking enabled in Fedora or RHEL anytime soon after it lands in mainline, mostly due to political reasons, thought I'd very much like to be surprised when the day comes.