Honestly cant remember if there was a retard feature or not TBH mate, as I didnt have nitrous on my car back then so didnt need to use it.

The mapping software was a bit clunky looking (its old DOS software) but fundamentally worked.

I doubt there is much support for them these days so I probably wouldnt recomend one TBH

Not sure why your megasquirt hasnt got that feature, but should be easy enough to add to it if it hasnt.