When I input Backsub in Mathematica 10.2 and press Ctrl+K after, I couldn't get Backsubstitution. My OS is Windows 8 32-bit.
Updated
I found following symbol doesn't auto-complete also.
CoefficientDomain
EliminationOrder
MonomialOrder
PolynomialForm


Backsubstitutionis not being offered as a hint on my system either (on 10.2 on Win7-64bit). I would suggest that you report that to WRI. As to the Ctrl+K part, that doesn't actually autocomplete a partial match on my system for other keywords either. $\endgroup$Reduce.minFileNameJoin[{$InstallationDirectory, "SystemFiles", "Components", "AutoCompletionData", "Main", "OptionValues"}]one doesn't get offeredBacksubstitutionafter one writesBac, but if one fully spells outBacksubstitutionone finally getsTrue/Falseselection box. $\endgroup$