Xavanic makes some good points, and I'll admit, I don't know stuns weren't reduceable below .5 seconds. But I'll even add to his argument, which is that it's nearly impossible to get 81% tenacity, and you certainly can't do it with the items and runes you mentioned. Why is that? Because most tenacity is multiplicative, not additive, sadly. I've pasted a copy of what someone else posted in 2016 about how it works below.
So basically, if the average non-garen tank or juggernaut were to try to maximize tenacity through runes and items, the max percent reduceable CC you can reach is approx. 26% on a 2 second CC. Less that 2 seconds, you the start reaching the "can't go below .5 seconds cc" problem. The sources for getting max tenacity at late game are as follows: Merc treads (30%), steraks (30% for 8 seconds), legend: tenacity (30%), elixir of iron (25%) for a total of 115% if you were to stack them addivitely. But since they are mostly multiplicative, the calculation for a 2 second stun is 2*((1-.03)(1-0.3)(1-0.3)*(1-0.25)) = .51.
Then, .51/2 = .26, or in other words, the total tenacity reduces CC only down to 26%, rather than capping out at 100%. Then of course, if you do the math for a 1.75 second stun, the efficiency would be even less since it can't be reduced below .5 seconds.
I don't know how this applies to cc that starts at .5 seconds or less though.
At anyrate, it's pretty easy to see that, along with other limitations, stacking tenacity is pretty garbage, maybe even more garbage than stacking armor pass 200. So it's just better to get only one form of tenacity, two at max, and then try get items or runes with more useful stats. So if you plan on merc and steraks, try to avoid a tenacity rune, as it will be wasted.
EndlessSorcerer (NA)
They are applied multiplicatively (not additively).
For example, suppose you had Mercury Treads (30%), Elixir of Iron (25%) and Tenacious (15%) and were hit by a 2s stun.
2s * (1 - 0.30) * (1 - 0.25) * (1 - 0.15)
=> 1.4s * (1 - 0.25) * (1 - 0.15)
=> 1.05s * (1 - 0.15)
=> 0.8925s
The stun would effectively only last 0.8925 seconds (45% of its initial duration).