This was first reported by OMG Ubuntu and It's FOSS, but it's something I experienced early on when I was testing the daily releases of Ubuntu 24.04.

The issue is how Canonical has secretly forced Snap installation on users. Previously, if you were to download a .deb file from the internet, the software installer GUI would open and prompt you to install the app.

That no longer happens. Instead, the file is downloaded and that's that. If you double-click the file, instead of an installer opening, the Archive tool opens, which is of no help to users.

To take this further, it looks as if the Software app defaults to Snap packages for everything now. I combed through various apps and found this to be the case.

I was able to find an exception with the Clementine audio player, which is no longer in development. When searching for that app, two versions appear – the Snap and .deb packages. But if you only search for Clementine and hit Enter, the Snap package is the only one you see. Run the search and wait for the drop-down to populate, and you'll see two different versions – one listed as a Snap package and one listed as a Debian package.

That means all is not lost for DEBs but you have to be a bit sneaky. As far as the auto-installation of downloaded .deb files, you'll have to install something like gdebi to bring back this feature.

It also seems (according to It's FOSS) that Canonical has no intention of fixing this "flaw" and will, most likely, continue to migrate Ubuntu until it is a Snap-only system.