Germany mandates them, they're variably permitted across the rest of Europe. They've been legal in the UK on microlights since 1998, but adoption hasn't been huge. They've become more popular since the UK microlight rules were changed to add 22.5kg to the permitted MTOW with a BRS carried.
Types most likely to have them in the UK are the Cirrus family, C42, CT, but there are others, even flexwings. There are mods for Cessnas as well.
You could potentially fit one to a light twin, but I don't think that anybody actually has.
I have seen them fitted "illegally" to UK registered aircraft as well, with no formal modification approval. I'm past being a policeman, but that does mean that there are aeroplanes flying in this country with explosive devices fitted which have never been through any form of safety approval system. I'm sure that'd worry me if I was a fireman !
Most usually look for a cylinder like this...
Or a marking on the outside of the aircraft, usually a large triangle, showing the rocket exit point. See page 21 of
this document.
G