Does the 80% capacity rule of thumb for ZFS pools apply regardless of the size of the pool? If I have a pool of 10TB, that means I have to keep 2TB free. Fair enough, the loss isn't too great. But in a 50TB pool, that means I need to keep 10TB free. That's a lot of free space left over...
Yes. The 80% rule applies... (with some exceptions)
Think about it the same way you'd approach monitoring of a volume. 90% full is 90% regardless of the actual capacity and would still yield an alert.
This is no different. You don't want to plan or expect to run at that high of a capacity.