Apple hasn't provided anything to measure web to app activity, regardless if you own it or not. For many "app only", it won't matter too much, for others it's a nightmare.
Most MMP are claiming that tracking web2app traffic through fingerprinting when it comes from your own properties is a legitimate use case, as those are first party.In my opinion this is a very lax interpretation to give devs a solution, but that's not great: besides the reliability of such method (especially on iOS), Apple has made clear that fingerprinting is not an option in any case without consent. That makes tracking web to app off guidelines including from your own properties. I don't think Apple will enforce this particular one from day1, but I wouldn't make big bets on it either.
For now, the only way to measure web traffic to the AppStore without ATT approved is through the very limited AppStore campaign links (with UTM, go to AppStore connect then campaign to see some data)