All 3 are very different in how their animation cancelling behaves.
Graves AA R E makes it so that he completely cancels the pushback - which made it so that Graves would have to use his ultimate as a finisher and not part of his combo because he can't really chase after ulting. With that cancel he completely circumvents this intended drawback rather than simply dealing more damage in a short amount of time.
Riven's animation cancelling is scripted:
"For the record, Riven is one of the few champions in the game that have intentionally scripted animation cancels."
Her Q cancel makes it so that the time in which Riven is locked in the Q animation is shortened so that she can use her AA sooner (her Q "resets" her AA timer). All it does is decrease the time it takes Riven to use her full Q+AA combo.
Another notable thing is her doublecast, it's essentially being able to W and Q at the same time after using her dash. This was not intended but is getting preserved.
Renekton's animation cancelling is less known so I'll just link a nice combo:
https://clips.twitch.tv/LongDoubtfulCookieJKanStyle
Most notable and importantly there is the W cancel: Renekton has a rather long animation lock when using his W in which he is unable to move. Using R/tiamat enables him to interrupt this lock. This is unchanged.
And then there is his E-QAA. After dashing Renekton is able to autoattack within the Q animation (see video). While it is very different to Riven's Q combo there is some similiarity to Riven's doublecast: using two different animations at the same time after a dash. A true equivalent would be if Riven was able to autoattack during her 3rd Q after her dash, but it's still sort of similiar.
Conclusion: it's very understandable and reasonable that the Graves animation cancel get's deleted as it completely negates a drawback rather than just increasing the combo speed.
Renekton's animation cancel and Riven's Q combo are very different, but there is some similiarity to Riven's doublecast that is intentionally being kept in the game despite not being intended. As Renekton's EQAA doesn't really do much more than just increasing his combo speed I do not think that it should get removed. He needs a nerf, but taking someething away that has been learned by mains should only happen when it's absolutely necessary - and in Renekton's case it is not.
EDIT: Apparantly they buffed Renekton's cancelling: https://www.reddit.com/r/RenektonMains/comments/6913tn/everything_i_have_found_so_far_gameplay_breaking/