All webservers (apache, nginx, litespeed, caddy) have keep-alive enabled by default. All browsers also support it. Whether it's actually used/negotiated on most connections, I don't know, and if you want to be pedantic you're right that it can't be default by definition since you need the header to use it. But I suspect it actually is used for most connections nowadays.

Multiplexing is the default in HTTP/2 and 3. So you're still wrong.

Keepalive is literally the default. You have to send an additional header if you don't want to use keepalive.