You are correct that, to the best of our knowledge, no pro-Palestinian protestor has thus far been deported to El Salvador.
The precedent here is what's important, meaning the government is arguing that they have the right to deport anyone they want for pretty much any reason and put them in a foreign prison indefinitely.
The legal justification for all this is an over 200 year old law called the Alien and Seditions Act and a declared state of emergency and invasion/incursion by a Venezuelan gang, something which has gotten at least some support from the Supreme Court [3].
Secretary of State Marco Rubio has basically come out and said that Khalil is being deported for his views. If these people don't have rights to free speech and due process then nobody does.
[1]: https://apnews.com/article/mahmoud-khalil-columbia-universit...
[2]: https://www.axios.com/2025/03/09/ice-arrests-palestinian-act...
[3]: https://www.livenowfox.com/news/supreme-court-lifts-order-bl...