Is it easy to bring an AG online after taking it offline.
That entirely depends on why the AG went offline. Taking an AG offline manually should pretty much never be an action that is done. There are reasons to, however those are generally after other bad things have happened and you're attempting to remedy the situation (whether those issues were internal to SQL or external).
Documentation says: After an availability group goes offline, its databases become unavailable to clients and you cannot bring the availability group back online. Therefore, take an availability group offline only to migrate the availability group resources from one WSFC cluster to another.
That's correct, there is a cross cluster migration but no one should ever do it. This is a hold over from a very long time ago (10+ years at this point) when there wasn't a good solution for upgrading an AG across clusters. Now every version of SQL Server in mainstream support has the ability to migration across clusters natively via distributed AGs.
"You need to set RegisterAllProvidersIP to 0 Thereafter take the availability group offline and bring online again."
Hard to respond to this because in a vacuum it's arguably misleading but with a specific use case it can be arguably mostly correct. The thing about "take the AG offline and bring it online" seems to be the major hold up here. In this above specific scenario, just do a failover as that will take the AG offline and bring it online on another node. I'm not sure why the hang up or wanting to bring an AG offline, randomly, and leave it that way.
So going back to is it easy to bring it online, then yes do a failover and it'll take it offline and bring it online. That's kind of trivial. Taking an AG offline and just leaving it that way I don't see many use cases for this.
