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).
I would say the easiest is to have the structure I just suggested:
Level 1: Class features, background and race, base numbers (HD, proficiency, etc.)
Level 2: Subclass feature (new class feature).
Level 3: extra HD, improve attack, saves, other numbers
Level 4: ASI/feat
Level 5: as level 3
Level 6: class or subclass feature
Level 7: as level 3
Level 8: ASI/feat
etc.
Class/subclass features at 1, 2, 6, 10, 14, 18.
ASI/feats at 4, 8, 12, 16, 20 (or 19 if you want the 5E-thing)
Improved numbers at 1 (base), 3, 5, 7, 9, 11, 13, 15, 17, 19 (and spellcasting levels)
(NOTE: how you want to spread out the numbers depends on the scale of the game systems. Off-hand I would suggest HD at every level, attack and saves together, and then skills and maybe additional feature uses together.
The compliation of "improved numbers" on the odd levels is strong incentive to reach those levels. On the even levels you gain a feature of some sort (or feat/ASI).
Like current multiclassing in 5E you only get a subset of the features available at level 1 (no base numbers). And you could get a 2nd level of another class to pick up the subclass, but now you'll miss number improvements until another level.
