My guesstimate is that the number of URLs that are shared on Facebook AND that already have a completely orthogonal "fbclid" parameter is infinitesimal.
Maybe among the URLs shared on Facebook there are a few whose servers only respond to a fixed amount of parameters, changing their behaviour when additional unused parameters are appended to the query string, but I imagine that the number of such cases is so low it's not even worth considering.
What exactly is Facebook breaking, in your opinion?
Would Facebook also break things if they were instead making an async request to the destination and appending a custom header to it, something like "X-Coming-From-Facebook"?
Maybe among the URLs shared on Facebook there are a few whose servers only respond to a fixed amount of parameters, changing their behaviour when additional unused parameters are appended to the query string, but I imagine that the number of such cases is so low it's not even worth considering.
What exactly is Facebook breaking, in your opinion?
Would Facebook also break things if they were instead making an async request to the destination and appending a custom header to it, something like "X-Coming-From-Facebook"?