For well over a decade many of us have been pleading for just such a opt-out switch, as we consider this to be a big invasion of privacy issue.  Here's another thread on this topic from two years ago: https://communityhub.strava.com/ideas/get-rid-of-suggested-athletes-who-to-follow-banner-5868  Feel free to add your vote to that idea... if you think it will help!  

Thanks ​@JBW-Florida for pointing me out to this other post. I will do a bit more research on my side.

The suggestions based on the fact that we’ve run / riden a segment together which is labeled as “You’ve done activity together” in the friend suggestions means that Strava is actively comparing location data from users who are totally unrelated and is saving this information. This cannot be done without explicit consent. This is Facebook level. See this story from 2016: https://www.splinter.com/facebook-recommended-that-this-psychiatrists-patients-f-1793861472

I would agree to have suggestions based on friends in common or common groups we are part of or a combination of both since this is community. But using location data is inacceptable, which is well illustrated in that article from 2016.

I did notice on race activities that I would be matched with a number of people I wouldnt know as “we’ve done this activity together” directly on the activity. This is something we should also be able to opt-out.

The strange thing is that we can opt out from having our data used in an anonymized fashion to help urban planing but we can opt out from having our location data used to determine who we were with, which may reveal sensitive information.