Does anyone have made any mod that will place, item restrictions to the classes?
I mean like daggerfall for example.

Example:
Mage cannot use any armor at all
Thief can wear only light armor (heavy and medium is restricted) or... more specifically, thief can use only armor of a specific encumbrance or... thief can use only fur/leather armor etc.

Does anyone knows how it can be made? I mean the logic / functions to make something like that?

There are several OBSE functions that allow you to inspect the equipment of the player (and NPCs). You can use http://cs.elderscrolls.com/constwiki/index.php/GetEquippedObject to find out what the player is wearing at the moment. Then you can use functions like http://cs.elderscrolls.com/constwiki/index.php/GetArmorType and http://cs.elderscrolls.com/constwiki/index.php/GetWeight to find out what kind of armor you're dealing with. Search the Wiki and OBSE documentation for the functions you need, and ask it here if you can't find a function.

It's actually not quite complicated to implement such things. But it'd be a huge immersion-breaker to not be able to use certain items anymore just because of your class.

I beg to differ. infact, i think that it would improve to realism imo. ie. a mage is too weak to use armor, a thief is only strong enough to use light armor.

In that case, I would check for attributes rather than the classes themselves. This allows for hybrid classes such as Battlemages to still make sense with the mod, and also un-complicates the question of how to deal with custom classes. (my two cents, spend as you will)

That's the biggest problem with trying to do something like this in Oblivion - there really isn't any such thing as pure classes. In D&D, you can combine classes, but you still have something that is part mage and part something else. You don't have that in Oblivion. I don't think there are very many players that use the base classes. I've never played a base class - even on my first play-through years ago. Even if you try to use attributes, if you play long enough, you'll max out all your attributes anyway. The closest you come to in Oblivion is to penalize certain skills while wearing armour, and even then the penalty usually isn't enough to prevent it. I always play a mage-thief-warrior - often wearing Daedric armour! So much for armour penalties

Well, you could look at more than just armor, and you could go further than the small penalty in the vanilla game - maybe as far as "full armor == silenced". I do like the idea of checking attributes and having penalties in one skill for levels in others.

You already have encumbrance limiting the amount of armor a weak character can carry (and so equip). Armor gets "lighter" with skill, but weapons don't, so a low-strength mage is at a disadvantage. But with the current setup, that mage can decide to work on skills that increase his strength if he wants to do so, without being artificially "banned" by his initial choices. The levelling system could be tweaked so that "opposite" attributes couldn't be increased at the same time, but I'm not sure that would make much difference. A few skill-to-attribute relationships could be changed, so that a different attribute is increased by leveling a particular skill, but there's already a fairly good distinction between the physical damage skills and the magical ones, so you couldn't do much there. To make a real change, you'd have to start decreasing the opposite side.

Bottom line is that I'd expect any change to be either ineffective, or too effective. You'd have to go to an extreme to make any change work, and that might be too far.