No, it was implicitly judgmental about the listener being at parties (i.e. your mother was expressing some amount of preference that you not go).

This is explicitly contrary to her explicit encouragement that I go to parties more when I was young. I didn't really go out very often at all.

Your mother told you not to accept drinks from strangers at parties? Do you recall her rationale?

Yes. No. As far as I can recall/tell, it was a simple precaution against getting drugged or poisoned or just getting sick from drinking from a glass that a prankster or otherwise ill-intentioned stranger might have put something in, or even from just getting sick from drinking from a glass that a stranger has also drank from or spit in or whatever.

If there were no such chemical as a roofie, would the no-stranger-drinks rule at parties be a good idea?

I don't understand the relevance / what you mean. If you mean if there were no harmful drugs or other bad stuff that could be invisibly inserted in drinks, then not particularly (the above reasons would still be valid, but not really worth having such a strong admonition for).

But this feels like asking "If physics never allowed car accidents, would the always-wear-a-seatbelt rule in cars be a good idea?"