"
Active: Kayn gains 40% bonus movement speed, is ghosted and can ignore terrain collision for 1.5 seconds. Immobilizing effects will interrupt Shadow Step.
Kayn heals himself upon moving through terrain for the first time and, if he's not in combat with enemy champions, Shadow Step's duration is increased to 5 seconds.
Heal: 100 / 115 / 130 / 145 / 160 (+ 40% bonus AD)
Entering combat with a champion after the effect is enhanced reduces the remaining duration to 1.5 seconds. The effect also cannot remain active out of terrain for longer than 1.5 seconds.
Shadow Assassin: Casting Shadow Step removes slows, additionally if Kayn enters terrain at least once, Shadow Step will be improved for the remainder of its duration to grant 70% bonus movement speed and make Kayn immune to slows.
Rhaast: Shadow Step is unchanged."
Emphasis mine. (From wiki.)
It's an awful lot for a tooltip, and after people grumbled about the last few champs having small novels in their tool tips I suppose they've opted to cut back.
I in general like long detail tooltips, but I feel like theres probably more people out there who see a big tool tip and immediately go tldr and don't even bother.