Alternatively there's http://www.msftconnecttest.com/ncsi.txt

I was always wondering given that firefox has that detectportal.firefox.com why they also put requests into example.com/example.org?

I found https://community.ui.com/questions/Apple-iPhone-iOS-portals-... which lists some of these domains (www.itools.info, www.thinkdifferent.us, etc.), and this answer on StackOverflow -- https://stackoverflow.com/a/22277933/ -- which reverses a list of domains that all point to the same IP