First of all, there is a difference between sending bounced off multi-tracks and audio stems to a mixing engineer.
"Audio Stems" often refer to a whole group of tracks summed together e.g. drums or strings in a mix.
Ideally mixing engineers don't mind you sending them recording effects as part of your sound because that, as a result, encompasses sound design.
Better yet you could also send mixing engineers your reference track which showcases what type of sound you are aiming for.
Worst case scenario is using an esoteric plugin so as to speak, which as a result a mixing engineer can't replicate a certain sound without it, and you not telling them you used it.
In other words, you can take image screenshots (where necessary), of...