By definition, 6dof requires all six control axes. If you can't roll, it's not 6dof. If you can't slide, it's not 6dof.
Sliding is surprisingly common, and doesn't make your game a 6DOF one - in slide (think: Freespace 2 or Wing Commander IV) you are locked on at least one axis (sometimes it's up to 3 axis lock - you can only rotate, but can't change your motion in other way than by exiting the slide mode), so you are still 5 or 4DOF game.
What space sims are missing most often from becoming a 6DOF games is strafing up and down. If you see someone talking about 5DOF game - it's on 99% missing strafing in vertical axis. 4DOF space sims are also quite common - Wing Commander games are 4DOF - they don't give you controls in either left/right nor up/down axis.
To give you an example on the actual games, let's take a look at the games Chris Roberts made:
Wing Commander IV (1996) is 4DOF game - no direct control over your ship in left/right or top/down axis
Starlancer (2000) is a 5DOF game - no direct control over your ship in up/down axis
Freelancer (2003) is a 6DOF game - you can control your ship in every axis of movement and rotation (even though it's mouse-oriented arcade shooter)
That's an interesting take on it, I suppose the argument could be made that it is! If that's the case though, I would think Star Citizen would also fall into that category right? What about other games with similar space-flight mechanics? I think it's an interesting debate Does anyone else have a view on this interpretation of 6DoF?
I don't know why it's even put in doubt and all the question marks.
It's obvious that Star Citizen is 6DOF game. It gives you a control over rotation along 3 axis and movement along 3 axis, all independently accessible at any time. Yes, you can turn on computer assist for flying, but you can still go for flying in full 6DOF controls.
^ Each color is a different axis of rotation or movement. 6 colors in total, for 6 DOF games.