Generally you'd have a small disk quota, and you'd get yelled at / disconnected if you used too much CPU or RAM, and you'd not be allowed to run processes while you're not dialed up. But other than that, you could compile and run stuff like slirp, sure.
Then ISPs didn't like people running SLIRP, so often there were "no SLIRP" rules. :P
Then ISPs didn't like people running SLIRP, so often there were "no SLIRP" rules. :P