UFC 1-4 allowed all:

. pressure points attacks
. strikes to: nuts, throat, eyes, back of head & neck down to the full spine
. small joint manipulations including wrist locks, thumb locks, bending & breaking fingers & toes, etc.
. all strike positions with elbows, knees, headbutts, etc.
. downed opponent, no problem with soccer kicking his head, head stomps, crashing knee to the throat, nuts, etc.
. all throws, take downs, suplex, piledriver, etc.

Not Allowed (but didn't DQ, just $1k fine): eye gouging (where it's not a strike but trying to scoop eyeballs out of their socket), fish-hooking and biting.

But since these didn't DQ and only $1k fine imposed per incident, the fight would be stopped and then restarted standing up which would allow someone caught in, ie. a choke hold to cheaply get out. Grand Prize was $60,000 (1993) which is about $93,000 today so that's well worth the $1k fine, even 10x and still come out ahead. I think fighters gets $1k/fight if they lose, and more if they win also.

Kung-Fu guy and a Judoka, both started to bite Royce Gracie when they were caught....which was why Gracie tried to break their arm after they tapped and the ref had to pull him off.