alt Hacker NewsZFS will auto-degrade performance if there isn't enough headroom. In addition, if you use SSD, you also want the headroom, because otherwise you end up writing and rewriting on the same small empty space which kills SSD (unless you have enterprise SSD, which have built-in headroom).
Not sure why I'm being downvoted for being factually correct (and also, why I can't edit my above comment).
ZFS will definitely degrade write performance gradually from 90% utilization, and will hit a stronger cliff at 95%+. Same with SSD, using a consumer SSD above 90%+ utilization would rapidly degrade its lifetime. The effect will be smaller for very large pools and very large files, but the effect is stil there.
I'm repeating this so that people who set up these drives know what is actually going to happen.