There is a single official CG in that shows Meltilith with legs going all the way down to just below the knee 

Its Called Melts Punishment. It has the most awkward posing so Im not gonna post it here.

(edit I checked it goes below the knee and then seems to just be covered by up by the metal, so at most shes a below the knee Plegic)
So...

She has alot of actual leg all the same. Just thought people should know just how much 'organic' leg mass melt has. 

(also turns out the legs are detachable so like... invest in some prothestics)

Meaning Ironically in that fanart image I shown, The Boots begin exactly where 'normal' organic legs end. Which means Melt can wear different high-length Shoes as long as they double up as prothestic legs

TLDR: Melt can wear different shoes as long as they go to just below the knee in terms of length and double up as prothestic limbs