Tiny Tools › URL parameter glossary › si
siA share source identifier that YouTube (youtu.be/...?si=) and Spotify add when you use their share button. It can link the share back to your session. Note: on YouTube it is not the timestamp — that's t=, which you should keep.
Safe to remove? Yes. si is used for tracking and reporting only — deleting it does not change the page you land on. Strip it before sharing a link to keep it clean and private.