Well, for Rupture, Zileas has stated:
I've been asked a few times, "Why don't you do stuff like Rupture (from DOTA Bloodseeker) in LoL?" I usually respond -- Rupture contains several basic design 'anti-patterns'. I thought I'd post for the benefit of those who are interested what strong anti-patterns I am aware of.
Literally the first anti-pattern one of Riot's chief designers points to as bad design. Linked the resk of his post in that quote.
For Kunkkas's ultimate - that's thing's pretty awesome, but it's also crazy powerful, and you're significantly more dead if he lands that on your than taking a rocket grab to the face.
It's also worth noting a solid ~1/3rd of the champions in LoL are pretty mobile, and if mobility is king you have a solid 1/3rd of the roaster to choose from.
If anti-mobi skills like the 2 you mentioned become useful for keeping mobile champions in check: you're narrowing the field from 1/3rd is a problem to 12 champions are a serious problem - which is not ideal for encouraging champion diversity or countering a vein of problematic choices. You're just bashing the same problem with a worse version of the problem.
As for mobility being an issue - I really don't see this mobility-creep issue everyone's clamoring about.
First - the most mobile classes in the game are the best public game stompers. When you'll only eat ~25% of your HP mid-dash, you're a rockstar in 1v1 scenarios yoloqueue constantly throws your way.
In a team-fight, eating 25% of your HP from 5 guys mid-dash is nowhere near as fun. Defense, range, and crowd-control are better defensive tools in a team-fight, and team-fights usually decide games.
Anyways - considering the sodium levels mobile champions squeeze out of the type of players that QQ, mobility being a problem is incredibly suspect right here. Cool beans the community thinks the ~47% win-rate Akali and Yasou and Lee and LeBlanc are broken and the ~55% win-rate Janna (that was 90% win rate in qualifiers and 85% win rate in worlds) is poor and helpless. But really guys - this is the internet we're talking about, and the intellect of people on the internet.
When a lot of people are clamoring thing X is happening, it's probably a safer bet it's not and the internet is being the internet.
Secondly - for mobility creep (power creep involving mobilty) to actually exist, the relative power mobility offers to new champions should be phasing out the relative power mobility granted to older champions.
Essentially - new champions should be getting faster and faster.
As Kassadin is one of the original 40 champions, has maintained being the most mobile champion in the game until quite recently, and has been phasing out newer champions due to his mobility - that's the exact opposite of power-creep, the exact opposite of mobility creep. The fact his mobility was recently hit (quite hard) is also the exact opposite of mobility creep. Mobility creep states champions where slow before and faster now, where making a champion fast then, slow now is the exact opposite.
It's also worth noting the mobility of new champions has more to do with the archetype they inhabbit requiring mobility as one of their core strengths rather than having it just slapped on classes that weren't mobile before. I'm not sure I'd really call 
and 
more mobile than 
and 
, 
and 
slower than 
and 
, 
and 
slower than 
and 
, 
and 
slower than 
and 
, 
and 
slower than 
and 
, 
slower than 
.
Considering 
needs to turn around and AA something within 550 range to evade things and 
can leap over walls and out of a rocket-grab that hit him whenever he feels like it, I'm not even sure I'd call Kalista contributing much to mobility creep (though she is pretty mobile).
Anyways - are champions really getting more mobile?
Or is the internet just using the buzz-words mobility creep to make their QQ seem like an objective thing?
is the only champion the OP mentions by name, and she's one of the original 40 champions (her mobility skill hasn't really changed in it's mobility from LoL's beta to now through her reworks).
being useful now is the exact opposite of mobility creep being a thing, what with:
mobility creep = newer champions are more mobile and thus better than old champions
Katarina is one of the original 40 champions the game launched with.
She's also been QQed about when I started playing when 
and 
were new releases, and her appearances at the pro-level have mostly been pretty questionable in the half a decade from then to now (while her QQ threads have remained more prevalent than most of the top tournament picks).
Finally - I'm not saying mobility isn't a problem and can't be a problem, but from what I've seen - the problematic champions within any level seem to have a stronger than usual early/mid/late game, defenses, cc, range, vision control, aoe, utility, speed, and mobility.
If anything, it seems solid champions have everything (including mobility) rather than just being mobile. Super-mobile champions have certainly not been top-tier just from a mobility metric and mobility metric alone.
I mean - look at 
The common trend there isn't mobile, it's they have pretty much every capability ever what with essentially having 7 spells rather than 4, and you'll tend to notice a lot of top-tier picks trend more to a jack(or jill) of all trades, unusually masterful of a few of them dynamic. Rather than just "is mobile" or "is not mobile".
TLDR
being strong is not mobility creep.
