The ISP named Free in France also did this a while ago.
It was fairly well implemented I think: separated from your network, bandwidth was limited (to avoid impacting the host), you could opt-out (which meant opting out of using the guest network), joining the wifi was automatic if you had a cellphone with the same ISP and it was the same "guest" network for all routers so in big cities, you could rely only on this to access Internet.
It was stopped a few years ago when they deemed cellular network was reliable enough to not need the guest network.
It was fairly well implemented I think: separated from your network, bandwidth was limited (to avoid impacting the host), you could opt-out (which meant opting out of using the guest network), joining the wifi was automatic if you had a cellphone with the same ISP and it was the same "guest" network for all routers so in big cities, you could rely only on this to access Internet.
It was stopped a few years ago when they deemed cellular network was reliable enough to not need the guest network.