Like the post above me mentions, X11 hasn't been disabled (and doesn't look to be needed).  X13 doesn't really bring much to the table past what X11 does, so very few coins are using it as a POW.  Most of the coins that are still X13 have very little buy support and are kinda dying out.  Most likely, if the current X13 ecosystem keeps up, it will be removed from the pool at some point completely (instead of up/down repeatedly).

As for how you'll know, either your miner will failover to a different pool (shown in sgminer), or I'll post something to the header of WP (and here).  Although again, X11 going away is very unlikely.