Will a SLX fit on a Shimano deore 2x10 rear triangle?
With 11 speed cassettes the compatibility issue is not the width of the dropouts or rear hub, it's will the cassette fit on the freehub body. Some 11 speed systems require a slightly longer freehub body to fit onto.

Shimano MTB 11 speed cassettes will fit onto the same freehub body as a 10 speed (which is actually the Shimano 'Hyperglide' freehub standard that has been in use for decades). It manages this by having closer sprocket spacing and overhanging the back of the freehub body somewhat. The overhang is possible because the large low gear sprockets required on 11 speed systems can clear the spokes.