Ironically, this is only true for prepaid SIMs. As a result, in some EU countries it's easier to get a month-by-month postpaid plan – sometimes there's no KYC at all for these...

When did this change happen? I’ve done local SIM prepaid all over Europe over the past decade, but not so much recently

It didn't. It's still up to each country. There's still several without mandatory registration.