It’s not outdated, it’s just differently intended. OTR you can use over any IM allowing custom clients. OMEMO requires support in the protocol.

OTR is better than inline PGP for that purpose, because of temporary keys.

So if you have a legislation mandating that a certain IM network or social platform supports open API for custom clients, you can use OTR over it, you can use inline PGP over it, but you can’t use OMEMO over it.