In Band In a Box 2017 for Mac I would like to see a feature where if you add a solo track to a song, the other parts will become quiter so that the solo track will become the main focus of the track. This would be really handy for me when mixig because I would be able to make sure that I know what parts come in and out of the song.

Maybe someone from PG can confirm this, but I'm pretty sure most of the dedicated "soloist" instruments already have been outputted at slightly higher db than rhythm Rt's, regardless of which track you actually place it in.
I for example frequently use the Medley function to switch from let's say a jazz guitar rhythm instrument to jazz guitar soloist during the soloing part of my song, and without needing to do any volume adjusting, I always get the sound of the soloing part a bit louder in the mix.
I know it's not precisely your idea, but seems to be the way PG has chosen to go to achieve much the same result.

Ok, I didn't l know if PG Music made a "ducking" feature in BIAB for solo tracks.

It seems to be, or at least the end result has somewhat that effect.
I think this was something they mentioned in the Features video for 2011 (possibly the odd 2011.5 update), if you're interested in looking it up.