What kind of bike is a brooks champion flyer saddle suitable for?
I assume you are talking about this page: https://www.sheldonbrown.com/saddles.html

I think what Sheldon is saying is that upright riding positions - with the bars level with or higher than the saddle -  would benefit from a sprung saddle. I don't see anywhere where he says spring saddles are not suitable for use with lower bars.

If you like the saddle and it works on your commuting MTB go ahead and use it.