I knew a lapsed Catholic from Germany who signed off (I don’t know the exact paperwork he filed, so it may not have an “apostasy”) - part of the reason was that there were taxes assessed that went to the church, but were collected from your paycheck. No church meant less taxes... I wonder if the same thing is done in Belgium.
Unless a person fills out a ton of paperwork and has a hearing in front of a group of priests/bishops/cardinals, they do not renounce their Baptism.
I don't know about Belgium, but I know it's done in Spain, in Austria, in Slovakia, and in Canada.
Canada does it more specifically to allocate property tax revenues into the Catholic public school system. Other countries do it because the church buildings are government-owned and therefore government-funded.