Snopes says the idea of asking for an "angel shot" to get out of a potentially dangerous situation is a mixture of truth and falsehood:
CLAIM: Women in a dangerous situation at a bar can order an angel shot to ensure safe passage into an Uber and away from a frightening man.
WHAT'S TRUE: A discreet sign posted at one restaurant in St. Petersburg, Florida, instructed women to order an "angel shot" if they felt they were in danger, or needed access help or to be escorted to an Uber.
WHAT'S FALSE: This practice does not appear to be widespread but depends upon a venue's staff being knowledgeable and trained to respond effectively, and abusers may know as much about the concept as potential victims.
Now that Snopes, Reddit, Redbook, and Good Housekeeping have covered this, asking for an angel shot probably won't work as well as it did, if it ever worked well in the first place:
The "angel shot" concept as broadly applied seems to have some less thought-through aspects. For one, issuing a clandestine call for help would generally only be necessary if a woman were within earshot of the individual she felt endangered by; otherwise, she could openly and directly ask for help instead of having to depend upon the chance that the bartender on duty knew the meaning of the "angel shot" code and that venue staff were trained to respond to it effectively.