" it can make it unclear as to whether any approach is completely off-limits, or whether it is sometimes acceptable"
That's because all people are different, yes? This board doesn't set societal rules unfortunately
There isn't going to be a rule that works for everyone. So if you want to be 100% certain of not inadvertently upsetting a woman, don't approach her on the street. If you want to be fairly certain of not upsetting her, approach her carefully, watch her body language, back off immediately if there is any vibe of discomfort. Put her reaction above your own interest.
Men can perfectly readily read body language in other situations (eg is now a good time to interrupt the boss? Make a judgement, pick your moment, leave it if it looks like a bad time, pull back quickly if you try but get a brusque response etc) - but most street harassment has NOTHING to do with thinking there might possibly be a mutual interest but getting it wrong and quickly withdrawing...