"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 though maybe my knowledge is out of date.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?

One problem with solutions like that is the the website needs to pay for every log in. So you save a few dollars blocking scrapers but now you have to pay thousands of dollars to this company instead.

Im from europe I never heard about it

In Singapore, we have SingPass, which is also an OpenID Connect implementation.