I just wanted to have a deeper look at the graphical modeller in QGIS 2.99.0-147, and I stumbled upon the providers not available under Settings|Options...|Processing|Providers as shown in the screenshot below (Good idea to move it away from the processing menu by the way...).
I installed it with the 64bit OSGeo4W installer on Windows 10, and it is installed as QGIS 2.99.0 with GRASS 7.2.1 (Nightly), so shouldn't at least provider 'GRASS' be available?
And where have all the others gone (e.g. SAGA)? Will they be there again in QGIS 3.0?