well, it’s complicated
A few “larger” cities have their own open data license, the 3 Regions also have their licenses, and there are a few licenses on the federal level as well. Most licences, however, are more or less rebranded CC-BY licenses (IMHO, for PR and/or NIH reasons, but that’s just my personal opinion…)
At the federal level, we are working on a Royal Decree that will promote CC-Zero(-ish) license in a comply or explain way: CC-Zero(-ish) as default, CC-BY(-ish) as second license - with a motivation why CCO wasn’t good enough - and custom license - again with an explanation why CC0/CC-BY wasn’t selected.
I’m not sure if CC-Zero / CC-By will literally be mentioned in the formal text, to keep it flexible and avoid having to update the decree in case the CC-Zero would be updated…
Anyway, if you want to discuss this in detail, I’d suggest to contact the legal expert of the Simplification Agency (DAV/ASA), Ms.Martine Trznadel and/or Mr.Noêl Van Herreweghe (soon-to-be-retired open data program manager of the Flanders Region) and/or his successor Mr.Mathias De Schrijver)