Thanks @SGJanssens for starting this post - I've had the Linguist page starred for a while but haven't gotten around to figuring out what needs to be done in order to actually get the Maple tag into Github.
I definitely agree that Maple should be there, especially given that there are so many obscure languages in the list, however, I also seem to recall reading that Linguist won't add a language unless it is in a certain number of repositories first. I'm not sure if they hold to that though.
I've been trying to keep track of any Maple work that I've found. So far, I've spotted these (this is certainly not an exhaustive list):
Hopf algebra of symmetric functions : https://github.com/bfauser/Maple-SchurFkt
schubert (Intersection Theory) : https://github.com/jmokland/schubert
ParametricMatrixTools : https://github.com/steventhornton/ParametricMatrixTools
polynomio (advanced computations on polynomials) : https://github.com/filiatra/polyonimo
Kronecker coefficients : https://github.com/catch22/kronecker
Delay Tools : https://github.com/sgjanssens/DelayTools (You certainly know about this one already!)
Ranlip - Multivariate nonuniform random variate generator with arbitrary Lipschitz density : https://github.com/gbeliako/RanLip (available to be installed from the MapleCloud)
GR tensor III : https://github.com/grtensor/grtensor (available to be installed from the MapleCloud(available to be installed from the MapleCloud)
MapleV - An Emacs package for Maple developers : https://github.com/JoeRiel/maplev
OEIS and ClusterAnalysis (My contributions to the list)
Regarding syntax highlighting: I'll mention that the work that I've got in Github is only for an older version of Sublime, V2 (to which I've now added a license). @John May has another Sublime definition repository for Sublime V3 here: https://github.com/johnpmay/SublimeTextMaple I've also recently stumbled upon some work on sytnax highlighting files for Visual Studio: https://github.com/yu961549745/VSCodeHighlightForMaple
I'm not sure if any of these are representative of the Maple language as a whole (I'm not quite sure exactly what the standard should be here). I'd be inclined to dig up the Vim syntax definitions and use those.
I'm not sure if the above helps or not, but I think it's a great initiative.