I don’t see anything wrong with sharing that sort of advice in a private feedback message. How it is received would depend a lot on how it is stated. If someone came across as being a superior host giving unsolicited advice to a clueless one, it would likely be resented.
But something like, “Hi XX, when we stripped the bed, we noticed the mattress didn’t have a waterproof cover. Both ourselves and other hosts who write in to the forums we follow have had guest situations where the mattress got ruined by something soaking through the bedding, or would have if there hadn’t been a waterproof mattress casing. It’s also really important in prevention of bedbug infestations, which any guest can unknowingly bring to your rental. Just thought we’d share that bit of advice to hopefully keep you from having to deal with such things in the future. And thanks for hosting us- we had a nice time and enjoyed our stay.”
They can either take it on board or not.
What I think other hosts tend to resent is when other hosts make a bunch of suggestions either during or after their stay that are just a matter of somethng the guest would have liked, but which the host doesn’t provide and doesn’t want to. That sort of thing comes across as “I do this, so you should too if you want to be as good a host as I am”. Like telling you that you should have a microwave or a toaster oven, or a Keurig. That’s different from suggesting a waterproof cover or saying that it was inconvenient not to have a reading lamp next to the bed.