Deprecated means that this still works but will stop doing so in a future release. Neos currently requires
"typo3fluid/fluid": "^2.7.0", and we are currently not working on integration newer releases of fluid.
Once other consumers of fluid start migration to fluid 3 we might think about that one day but not before that. Afaik even projects that rely on Fluid heavily are still discussing that.
For me personally fluid is more like a legacy system we still support but rather would make optional on the long run. We probably will support this for a long time still but unless someone shows up willing to put work into this we will merely keep it running as it is.