Well, not gone. They are listed but when you click on the download button the 404 page pops up. I didn't try every version listed, but about 5-6 before I figured out something was wrong.
Is that to download the old version so the app? Because we already keep them in the releases page: Releases · laurent22/joplin · GitHub
OK. But I just Googled "older versions of Joplin app" and the first entry to come up was the link to Uptodown. It has all the Joplin apps listed, but nothing happens when the download button is clicked. Thought you might like to know.