Stricto sensu nature reserve are boundary=protected_area in Brussels : 

https://document.environnement.brussels/opac_css/elecfile/sols_14_.pdf : 

"It (active protection)
applies to sites of very high biological value requiring strict protection such as
natural or forest reserves and Natura 2000 sites"

it's probably a good idea to add the protection status  in OSM because in Brusslels there's some of leisure=  	nature_reserve are in law not protected :  https://www.openstreetmap.org/way/397630203 for exemple, but in fact can be considered like nature reserve.  

So add this proected info is usefull IMHO.