Depends on what you mean by restricted, but my point is that 3e/5e style multiclassing (stacking levels of multiple classes and gain the features of said class) does not allow for dead levels in design because any class with large amounts of dead levels will be seen as a signpost to drop the class a go on to another. For example, the 3.0 ranger has four dead levels between level 1 (which has a major drop of features) and level 5 (where spellcasting and second favored enemy goes live). Those four dead levels do give you stuff (BAB, HP, skill points) but you can get most or all that stuff by being a barbarian and fighter AND get the abilities of both of those classes (rage, bonus feats) rather than those dead levels. 5e tries to solve this by forcing you to give up far more stuff than those for dead levels. The cost is that you have more stuff to worry about. But as long as you stack that ranger and fighter (or barbarian, or both), you will always encourage people to jump classes.
You can put all manner of barriers in the way, but I tend to find that as long as there is a viable path, someone is going to take it. No amount of time, training, XP penalty, or other restriction will dissuade it unless it is so character crushing it is a trap build (and then, that's the same as banning it but with extra work). So the only way is to handle multi-classing differently than 3e/5e currently does. You could go with a 4e or pathfinder system where you could use a feat to poach a specific or limited class feature or something to that nature, or even a gestalt/AD&D style advance in two classes system (though that has its own issues).
