There is some tracking going on with twitter posts especially. Many have this s=21 at the end like this pb notable:
https://twitter.com/barackobama/status/1104910353232789505?s=21
The person(s) posting the s=21 is doing it intentionally. The s=21 marker in particular appears often and has no reason for being there except to allow twitter to track the link that was posted here.