Almost all software in Harmattan is a deb package, including pre-installed apps.

The problem is, to simplify updates Nokia made a giant metapackage (mp-harmattan-00x-pr, number depends on firmware version) that depends on almost everything in the system. Force removing, say, Twitter, will break this metapackage, it gets removed, and the packages it depended on (which are marked as automatically installed) are autoremoved. Half the system disappears if you confirm. Don't do this.

To uninstall these packages without causing breakages, editing /var/lib/dpkg/status and removing them from the dependencies of whichever mp-harmattan package you have should AFAICT (I have not tried this) allow the package to be removed without trashing everything. Removing dependencies is usually a Very Bad Idea, but as it's just a metapackage it shouldn't really make any difference.

Alternatively, you can go without the metapackage entirely and mark the other packages as manually installed, allowing you to remove anything that another package doesn't depend on. This does mean OS updates might not work correctly, but it doesn't look like there'll ever be another, so it's your call.

Of course, either solution doesn't mean removing bits of the system won't break things, it just means it won't do it for no reason. YouTube and Twitter apps should be fine to remove this way.

there is tweak in n9qt for freeing system packages and make them available for uninstallation.