"In Europe" is technically true but makes it sound more widely used than I believe it to be... though maybe my knowledge is out of date.

Their website lists 24 supported countries (including some non-EU like UK and Norway, and missing a few of the 27 EU countries) - https://www.itsme-id.com/en-GB/coverage

But does it actually have much use outside of Belgium?

Certainly in the UK I've never come across anyone, government or private business, mentioning it - even since the law passed requiring many sites to verify that visitors are adults. I wouldn't even be familiar with the name if I hadn't learned about its being used in Belgium.

Maybe some other countries are now using it, beyond just Belgium?

Oh I wasn't aware of that. I remember a Dutch friend talking to me about a similar app they had. Maybe they have a re-branded version of it?