Email, as far as im aware there isn’t some alternative email standard (messaging services, whatsapp, signal, sms, etc do not count imo as I believe they serve a different purpose than email)

DNS, while there are alternative root servers, they still fundamentally rely on the dns protocol.

TCP/IP, when the internet was first starting, this was not the only standard in use, but now it is (to my knowledge).

I thought about this for longer than I should’ve for a comment on a random post, but this is all I could think of lol.

edit: grammar