I don't know where this rule came from, but is bad and imprecise.
For HDD and blockstorage, performance will suffer if you fill your pool above 50%.
For SSDs and blockstorage, fragmentation will not hinder performance that much, since there are no heads involved. Maybe you have some pseudo SLC cache that will suffer.
For HDD/SSD and datasets, there is no reason why you should not be able to fill up your 80TB pool with 79TB of data.