Its an optional soft fork so since its optional in only some wallets it basically gets no adoption. It must be a protocol requirement for it to be standard, similar to ring-signatures on Monero.

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Yeah, at least on Cake wallet payjoin V2 requires the sender and reciever to support the feature. With v2 both don't need to be online at the same time, v1 required that both be online to process the transaction so even more niche.