How could I map a permanent food truck that does not have outdoor seating but bar tables? I did not find a “outdoor_bar_tables=yes” equivalent. One could easily introduce it, which leads to the question: What is the correct British English term for this:
Just to be clear, that’s a table but there are no chairs? Is the idea that you stand up while eating but have somewhere to put the food down between bites?
Yes, exactly.
In this case the equivalent to outdoor_seating=yes would be outdoor_standing=yes without doubt … 
There are indoor_seating=bar_table
I would in your case insert outdoor seating:comfort=bar table
https://wiki.openstreetmap.org/wiki/Key:outdoor_seating:comfort
where of course “seating” 
Hm, I see. In the sense that “outdoor_seating=no, but at least bar tables”.
I’d assume outdoor_seating as default for a food-truck… but anyway I wouldn’t tag movable tables/chairs at all.
In Street Complet there is the question: Where can you sit here? One of the answers is, "No seats, but not just for takeaway. = outdoor_seating=no, there is no takeaway. So it’s actually clear that I can eat my food there.
Some restaurants in my area remove chairs from their outdoor seating area every evening. Not sure if it’s a law or if they are worried that the chairs will be stolen.
If the non-fixed objects are expected to be present every time when the, e.g., food truck is present, I would map them (not separately, of course).
That’s too implicit for my taste. It also does not allow for the situation of “takeaway=yes” & “outdoor seating=no” & “bar tables=yes”.