Curse voice was originally showing the timers we all know and enjoy currently but long before everyone had access, that and only that was why it was deemed unacceptable. Since that was decided the program was rewritten to comply and no further issues have arisen. Using voice chat is not against the terms of service, that's like saying I can't call my friends or be in the same room with them when we play a game together (something every LCS team does is sit near each other and voice communicate). As long as the voice chat you are using is not giving you something extra (like timers or other data otherwise unavailable) it's fine.
[slayer-pantheon-popcorn]
RED POST
dArtagnan (NA)
This has been a tricky topic in the past, but I will do my best to answer it:
The short of it is yes, it's ok to use; using Curse Voice will not get you banned. However, 3rd party programs sometimes implement new features that do violate our terms and guidelines. Even if you don't use this feature specifically, it can get your account penalized. Just keep an eye out for any features that may give you an unfair competitive advantage in game.
Hope this helps! Let me know if you have any further questions!
http://boards.na.leagueoflegends.com/en/c/help-support/80wji5tw-curse-voice