My Opinion
I’ve been a bit confused why I see videos with that gesture censored, I feel like that rarely happened in the media I consumed a decade ago. However, I believe this “phenomenon” is older than TikTok and its weird censorship.
I’d place giving the finger at a high 2, low 3 on offensiveness, definitely nowhere near gesticulated sexual acts. You can give a child shit for using it, but the occasional finger is something I’d expect from even a teenager.
I’d say it really depends on context so it can be anything. Sometimes it can even mean something positive if it’s with a friend group for example. But if I were to think about the offensive context specifically I would go with 3. Personally I interpret the gesture as silent “fuck you” or “fuck off” which in itself is not that offensive either because of popular usage.