Just my two small cents: soft keywords have a cost as they make everything around them more complicated in
the parser. For example, creating custom error messages around soft keywords is one or two levels of magnitude
more complicated as sometimes you need to parse segments of syntactically invalid code, with some generality
(like "starts with this token and then anything can follow until this other token"). Soft keywords also make
highlighters' life more complicated as it has already been discussed.

And just to be clear: I am not saying they are bad, just that they are not free of cost.