The vast majority of people I've spoken to on the topic outside of the internet tell me that they/them is appropriate unless you know they do not prefer to be referred to as they/them. Basically if you're referring to someone you don't know pronouns for and don't have an opportunity to ask them...then they/them is fine. If someone has a nametag, or you have been told their pronouns, or you have an opportunity to ask them, then use the pronouns they've told you. It's kinda doubly confusing because at least in English "they" is never a gramatically incorrect pronoun...but just because you're gramatically correct if you refer to everyone as they/them all the time doesn't mean you should. Basically make the best possible effort you can to positively confirm someone's pronouns before referring to them with any pronoun first, and if you can't positively confirm someone's pronouns first, then they/them is usually the best option. Maybe this is just because I know a whole bunch of folks who prefer they/them, so their opinions on it are colored by their own identity.

Not everyone agrees with this though. I've called someone that I had no possible way to know the pronouns of "they" once in public and been snapped at by someone who wasn't even part of the conversation that did know this person's pronouns, and, okay. I made my best effort, it wasn't good enough for that person in that instance, so I apologized, corrected myself, and moved on. You're not going to get it right for everyone 100% of the time in all instances, it's just impossible because there is no 100% agreed upon convention that works in all languages at all times. Make the best possible effort you can, and if someone calls you out on it, don't take it personally, apologize, correct yourself, and do better next time. Even in this thread I won't be surprised if someone says I'm wrong because I missed some nuance or forgot about some circumstance that is important to their experience. That's fine, I'll learn something and do better next time.