Crippling slow effect appears to be 20% or 25%, I didn't do enough testing to get a good indicator which one was more likely.
As for Tendon Slice, I was able to get it to work and not work (more below), and suspect anything that is not Tendon Slice 12 is broken. Whilst Tendon Slice 12 items work without issues, or at least the one from the Legendary Cloak of the :: edit :: Forests Blade (Misnamed item) :: /edit :: works at least (item I had).
Tendon Slice in my test popped up from Tendon Slice 12 for :: edit :: 11 seconds (not 10 as I listed) :: /edit :: seconds on Elite difficulty (reduced in Reaper). Slow down effect appears with the overhead icon, an examination window buff/debuff icon, and a red mist around their feet indicating they have the Tendon Slice effect in action. This was tested with Handwraps without Vorpal Strikes (Bludgeon only), as well as with Blades (Daggers and Longswords), so the weapon type or its original damage type shouldn't impact on it working.
I also went and tried it on the Legendary Wallwatch Circlet (Tendon Slice 10) and it would not Tendon Slice anything, mirroring your experience. With ranged or melee weaponry (so not type specific like the Crippling effect which can be ranged or melee only). Suggesting either it's Tendon Slice 10 that's got the problem, or only Tendon Slice 12 is working. To make sure on the ranged toon that it was not anything else overriding the slow down effect with a superior one (Force version of the Shiradi Mantle), I disabled that during the test also.
I then tried two more items; Epic Memories of the Mighty (Tendon Slice 9) and Legendary Bottle of Rum (Tendon Slice 11). Neither of those worked too. Suggesting that all Tendon Slice is non functional bar Tendon Slice 12 at the moment.
J1NG
:: edit ::
Video of the Tendon Slice effect working.
You can see that it works for 11 seconds (not 12, but functionally 12 seconds). There's the sound indicator (turn sound on), and the other effects I mentioned. You can even see it move (more slowly) for a brief moment when it did move (a tiny bit).