The most common scenarios around here are that the road signs say either
- cycles, buses and taxis only, or
- no entry except cycles, buses and taxis
The word busway isn’t used so the “duck test” doesn’t work.
If there’s no hard line around either taxis or bicycles being allowed, then it’s less clear me when something should in theory be a busway or something else + access tags. (Practically, until busway is rendered on the ‘main map’, of course no one is going to go and retag the city’s most well known street to busway)
In any case, it should be possible to improve the section of the Wiki that says “don’t use highway=busway for transit malls” based on this discussion 
I’ll try to come up with a better phrasing and put it up for discussion here.
It also looks like there are some differences between countries so maybe it’s easier to get consensus in the local community on this question than globally.