Wikipedia (https://en.wikipedia.org/wiki/Foot_stove) says its restricted to wooden ones with hot charcoal, but I would call them “Foot Stove” regardless of construction material or heat source.
Never seen one of these, but my dad used to have charcoal hand warmers similar to this:
https://www.amazon.co.uk/Pocket-Hand-warmer-charcoal-includi...
We used to take them to football matches but eventually they became the sort of thing they'd confiscate cos fire and idiots don't mix.
Though I guess a microwavable grain bag on a normal footrest would also work.