They add one ad at the beginning like “visit example.com” inside the subtitle file

But luckily there aren’t many companies that want to be associated with piracy* so in most cases it’s blank a generic “advertise here contact mail@example.com”

I remember only a campaign that lasted over a year, a service that promotes legal streaming services, filmamo.it

* note: why I said “associated with piracy”: those files are going to be used with pirated video files and in most cases are pirated too as they’ve been extracted from a DVD or a streaming service without a license from the copyright holders. Once I saw a guide “how to use external subs with Netflix” for the rare edge cases where the viewer is watching a content via VPN and his sub language is missing, but those are very rare. Not to mention that for MPAA there’s no difference between just torrenting a movie or paying a subscription to Netflix in another country via VPN: for them is all piracy and they want to stop both.