As the other person said, they've always been allowed. Thieves and assassins both lose hide, assassins lose assassinate and their ability to stalk. Some things that only work in conjunction with hiding will not work with the Outlander chameleon power (which both classes get later than any other class that can join Outlander), and some things are still buggy from forever ago. It's a tough route. If you're going to do it, look up other Outlander assassins and see what levels they get chameleon so you can plan accordingly. Personally, I've tried not to be more than 5 levels away from getting the chameleon skill when I join so that I'm not a sitting duck forever.

Assassins and thieves both have always been able to join outlander. They both lose hide and assassins lose assassinate on top of that. I have heard rumors it's possible to get them back if you're a good roleplayer.

I forget the specifics of which, but Outlander assassins lose some skills, and I believe hide and assassinate are among them.