I'm also facing this issue. The server is already on TLS 1.2 since a few years back.
I checked that both the URL to the plist file on the download link and the URL of the software-package inside the .plist are https.
I also tried rebuilding the app using the xcode 12 to no avail.
The link still works on previous iOS versions and when I updated my iPad from 13 to 14.0.1 the installed app was still working properly, but when I deleted it I couldn't install it back.
What else could be the reason for this?