I think the rule "if it's not considered legit in a physical office, than it's not considered legit in an online meeting?" is not a good or logical rule, because it just are two very different settings. Some things are a major nuisance to others in a physical meeting while they are not in a online meeting. As Laurent S points out, smoking is a good example of this because the smoke and smell will not bother other participants in an online meeting. Other examples you give like visible shaving, cutting nails, dental flossing because it is (generally) more considered the sight that is distasteful. 

So I think until we all have a gizmo on our desks that transports smells, it is okay to smoke, be smelly or fart while muted in an online meeting. However doing visibly distasteful things will always be a no-no during an online meeting. Unless the cameras are turned off off-course in which case only audible distasteful things will be forbidden.