Is there some decentralized message system in actual use which I can offer as an alternative to entering an e-mail address?

I'm trying to be nice to my customers. I want them to be able to use my service without having to enter an e-mail address. However, I've been hunting myself for ages for any kind of "e-mail replacement" yet never found one. So I have no idea what I could reasonably let them enter as an alternative to having to enter an e-mail address, which I can then reliably use to send them notifications, such as:

Your submission has been approved!

Or:

Your ticket has a new reply: blablabla...

It should be something which doesn't require me to have registered an account somewhere. I'm imagining that they open up some kind of application similar to Bitcoin Core, but instead of "Generate new receive address", they "Generate new communication address" and then enter that unique string into my form instead of an e-mail address, so that I can reach them (for as long as they want to keep the address) without using e-mail.

I tried to understand how to implement the "browser notifications" thing a few years ago, several times, but found it to be incomprehensible and centralized. Did anything ever come from that? I just remember it as very annoying when sites kept asking for permission to send those things.

Is there such a thing yet? Which is worth supporting at all? I kind of gave up on "Bitmessage" long ago...