Is the Ultimate Hat rune restricted by the cdr cap?

As others have said, it's not restricted by the CDR cap.

However, it stacks multiplicatively with normal CDR, not additively.

So if you have 40% "normal" CDR + Ultimate Hat fully stacked, you get a grand total of 1 - (1-0.4)*(1-0.15) = 49% CDR on your R, not 55%.

In other words, if you have 0% CDR, UltHat really gives you 15% CDR on your R, but if you already have 40% CDR, it only gives you 9% for your R.

Ultimate Hat is not. It has been revealed earlier. There you go https://boards.na.leagueoflegends.com/en/c/developer-corner/3THTrhmb-runes-corner-late-game-focused-runes

Ultimate Hat is NOT restricted by the CDR cap.

I'm a bit late and i know it but does ultimate hat works on udyr u know the phoenix?

It gives 15% CDR on top of 40% from items.

It gives 15% CDR for your ult, beyond the normal 40%/45% cap.