My watch is counting steps while on a bus. I was suggested to turn off activity tracking. But there's no such option on my watch.

I can't answer why you don't see Activity Tracking in the menu, but I don't think that turning that off is the best solution. Really easy to forget to turn it on again when you need it. Easier is to start an bike activity and when you get off the bus you just stop and discard that activity. To save battery you can create a copy of the bike activity profile, name it "Bus ride" or similar and turn off GPS in that profile. Just remember to discard the activity to not mess up the bike stats.