(BTW, I went ahead and split/merged topics)
Based on what I've seen (and to some degree participated in, such as at WMAW), it really comes down to the participants fighting safely. That is, controlled, without too much power, and not intentionally thrusting into the eye slots. Some people use perf-plate or mesh inside the eye slots, but that seems to be in the minority. When they go for throws or general wrestling, they ease the other person to the ground, since the fall can really hurt. Safety matters more than winning.
So far with our demos, we have a lot of participants who haven't been doing Harnessfechten very much outside of the demos themselves, and admittedly we can ramp up the power at times. Also, when you can barely see your opponent, the armored fights can feel very chaotic. So at this point, we've favored "definitely won't go through the eye slot" over other considerations. That doesn't mean it has to stay that way, or that it's a perfect solution by any means.
At VAF, we did it with steel as well, but it was relatively slow and controlled, didn't have mordschlags for the most part, and focused more on point control. We were able to do it with visors open if we needed to.
Historically speaking, the visors weren't always used in foot combat. The manuscripts do show them used in duels, but there's also reasonable evidence that they were favored more for horseback, and dropping down to on-foot often meant raising the visor, or discarding the helmet, so you could actually see your opponent.
This of course is a problem we face in all aspects of WMA/HEMA-- There is no way to have a 100% perfect simulation, because we aren't killing each other with sharps. Any training weapon, any rule set, and any safety consideration will always introduce artifacts. It becomes an exercise in choosing which compromises are acceptable and which are not.