We have a ticket tracking this feature right now. I couldn't say if it used to exist or not, but if so it's before my involvement with Brewer's Friend.
Right now we're still in a discussion phase, with some more research needed. Unfortunately it's not as straight forward as most of us think of it. It's also decidedly overstated in some ways, there's a minimal difference in attenuation for say 148F vs 152F, but the difference between 154F and 160F is much more significant.
Other software that I'm aware of that has different versions of this feature are rather rudimentary, and are not consistent with each other or with actual brewing research. For example, certain software uses the same fermentability function for every recipe and every yeast as a function of mash temp. IE every recipe will lose ~2% attenuation per degree F as the mash temp gets further from a central point (usually ~150F).
This is not realistic as different yeast will have different fermentability vs mash temp curves, if the yeast attenuation range is from 67-77% attenuation (Omega OYL-003 London Ale), it will have a different curve than say a saison with 76-80% attenuation (Omega OYL-027 Belgian Saison I).
