downhill
Installing debs is absolutely not recommended. As Justin pointed out it is a unsupported use at your own risk alternative to simply extracting it to /
If you just extract it to / depending on what you are attempting to install it could overwrite important files, install to the wrong location so it does not integrate with Solus and/or be built against dependencies Solus does not have / has different versions of so it will not work at all. On top of all that it also can not be tracked by the package manager for removal so removing / updating it is a PITA for the user.
Solus does actually repackage .deb's to .eopkg when building from source is not possible and upstream only provides a .deb https://dev.getsol.us/source/opera-stable/browse/master/package.yml But as you can see we don't just extract it to / and let it do what it wants.
