When you double click on a deb package in Ubuntu 23.10 an error appears to tell you "there is no app installed for 'Debian package' files". In this post I
The default beaviour shouldn’t be to run it, but it also shouldn’t be to tell you that a program that can run it doesn’t exist, when it actually does.
If you want to do it via GUI, default behaviour should be to tell you that for security reasons, installation of deb files from the web is disabled, with a link to the root-accessible setting that enables it (similar to Android, for example).
The default beaviour shouldn’t be to run it, but it also shouldn’t be to tell you that a program that can run it doesn’t exist, when it actually does.
If you want to do it via GUI, default behaviour should be to tell you that for security reasons, installation of deb files from the web is disabled, with a link to the root-accessible setting that enables it (similar to Android, for example).