You can read some of the issues people have had with this by reading up on the http referer header: https://en.wikipedia.org/wiki/HTTP_referer

There are a lot of reasons I might not want a site to know where I came from to get to their site. It is basically sharing your browsing history with the site you are visiting.

Because of this, there have been a lot of updates to the http referer header, with restrictions on when it is sent, and an ability to opt out of the feature entirely.

Adding a url parameter with the same information bypasses any of these existing rules and ability to opt out. They should just use the standard.