Those first few days foiling were so scary, once you experience the taco/switchblade wipeout and are nearly certain to see it again. It happens so fast, you don't even have time to block it. For the next few years, it's pretty rare towards never, you learn not to do it or when you wipeout you know to jump away. Where I've had a few scary moments lately is trying to get to next level, committing to dropping in on bigger, steeper waves ... when you wipeout, you're right in the mix with the foil. I got pitched and had foil hit me in the head, stitches, and luckily nothing more (and no, I was not wearing my helmet).
So, for a beginner, I think face protection is not a bad idea at all for first few sessions. For intermediate / expert, probably the risk is manabeable to point that maybe not worth it.
From other threads where people talked this one option was kayak face protector:
www.outdoorplay.com/paddling/paddling-gear/paddle-helmetsAll you really need though is a single bar across football style, or like this:
www.thelasthunt.com/products/poc-artic-sl-spin-helmet-llll-poc-10497?gclid=Cj0KCQiArt6PBhCoARIsAMF5wajABZ4tCtmgCi0Q3C-RA4JR-qIZF85NMqvv0Azn4IIYd7WuZPxO1E8aAmcqEALw_wcBCould probably jerry rig an equivalent with an aluminum tube bolted on.