If I attack with Yasuo's Remorseful trigger, but on resolution of the trigger my opponent isn't there through something like Reaver's, does Yasuo's attack trigger's damage follow the unit?

No, Yasuo, Remorseful's attack trigger does not follow the unit.

According to the ruling in FAQ #6924 and FAQ #4040, Yasuo's ability requires the target to be "here" (at the same battlefield) when the ability resolves. If the target unit is moved away (e.g., by Reaver's Battlefield) before the trigger resolves, the ability will resolve but deal no damage because the target is no longer at the required location.

Sequence of Events:

1. Attack Trigger: Yasuo moves to the battlefield and his attack trigger is placed on the chain, targeting an enemy unit at that location.
2. Defend Trigger: If the opponent uses a defend trigger (like Reaver's Battlefield) in response, it is placed on the chain.
3. Resolution: The defend trigger resolves first, moving the targeted unit away from the battlefield.
4. Yasuo's Trigger: When Yasuo's trigger attempts to resolve, it checks for the target "here." Since the target is no longer at that battlefield, the instruction to deal damage cannot be followed and is ignored (Rule 356.3.e.6).

As noted in FAQ #4917, Riftbound abilities do not "fizzle" in the sense of failing to resolve; they always resolve, but they may result in no effect if the conditions (such as the target being "here") are not met at the time of resolution.