I know it isn't necessary but I don't believe it is a tracking token because all YouTube tracking tokens start with that. On my phone I can double tap the tracking token and it will delete everything before the ?si=. I used to delete the whole thing before but there was one time that I accidentally deleted one additional character and ended up posting a broken link haha so I figured just leaving the ?si= there is harmless. There is no tracking information.
Discussion
Obviously there is no official information what exactly the tokens represent but some people say ?si is a tracking token to, I also think as its a id not necessary to locate the video it has to have some other, undisclosed purpose. Better to get rid of it imo.
I worked in digital marketing a long time ago and I remember how Google analytics would add unique trackers to links in emails and social media pages. The ? and & are usually what is used. When the link is accessed, Google analytics stores the text that comes after the ? or &. The si stands for share info. But since there’s no text after, no info is shared. It’s like me giving you a blank piece of paper and telling you to save the information on it.