Some cafes have the shitty practice of imposing a captive portal for Internet access. Sometimes they demand personal information, and sometimes the captive portal discriminates against people with older phones.
Currently these cafes have the field “Internet access: customers”. That’s misleading and unjustly described. Some of them should be tagged with “Internet access: only for customers with new phones”. It’s not really fair to say it’s for all customers when they use exclusive technology.

I think you’re misunderstanding me. What I’m saying is literally “go for it”, and proposing what I think is the best way of doing it.
You’re free to just start adding
internet_access:captive_portaltags with whatever values you want to cafes you’ve surveyed, as long as you don’t breakinternet_accesstags it’s fine. OSM tags are folksonomy, after all. I think if you care about it this is the least you can do.Getting it on the wiki would be more difficult - it would require a detailed proposal and an RFC. That’s good in a way, it would force you to think about different values this key might have, and how consumers might handle it. You should probably also do this, so if others also want to do the same you can agree on which keys and values to use.
The most difficult part is going to be getting any data consumers to handle the key in any way. Unless you go ahead and make good-quality patches for a lot of apps (and have the energy to push them through), the best you can realistically hope for is for the key to show up in some obscure menu of “other tags”, likely not even searchable. This is the main reason why it’s going to be pretty much useless for the near future.
Believe me, I’ve been on a few cheap trips like that when I was younger. Back then very few cafes even had public WiFi, and despite
internet_accesskey being already defined, the easiest way to find a cafe with internet was to just walk about - almost none of it was mapped. You can expect the same to be true for your proposal for a long, long time - at least a decade or so, even if it gets accepted.