It’s worth saying that at one point OSM had more green tints than you could shake a stick at. There were numerous problems in terms of usability, maintainability & readability. Even now for biotopes there is heath, woodland and scrub, meadow, grassland, grass, bare rock, scree & shingle, sand, mud etc, which is quite a lot for what is general purpose.
One of the other problems is that many biotopes are not mapped in terms which a biologist or naturalist would recognise. Heath often covers upland grassland, blanket bog, and a range of other biotopes (although not altogether inconsistent with the EURINE definition), meadow is largely not meadow but agriculturally improved pasture land, grass is often used for urban commons (i.e., amenity grassland), but can be used elsewhere etc, etc. Personally I think the practical limit is around 4, possibly 5 shades of green, so additional symbology is necessary & can add a great deal of information: see by notes on Woodland Cartography & the level of detail we can go into for what is apparently a simple change.
It would be lovely to have a map more focused on biotopes, which would include things like contours & hypsometry. Representative examples which make a start are : OpenTopoMap and Outdoor maps from ThunderForest & MapBox.