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I agree about saying goodbye to ICQ. It's dying out.
I disagree with Skype – it's ICQ #2. After Microsoft took over, Skype could have been forgotten immediately. But out of habit, some people are still clinging to it. Those who don't want to bother installing Discord (I'm one of them; I only recently installed it because of Game of Thrones and am now happy). Now, Skype is just as useless and unnecessary as ICQ.
Options directly related to phones don't even need to be considered.
In conclusion, I support Slack or Discord. It doesn't matter which one specifically, as long as it works.
Two key actions:
(1) Once an authoritative leader is determined, create a proper guide for everyone on how to install it.
(2) For the Creator, it's time to add Discord and Slack to the profiles on the forum (there's currently ICQ there, along with completely unused AIM, MSN, and Yahoo).
And move everything else below. Specifically, move the three unused options from the top to the bottom, where Skype, Viber, WhatsApp, VKontakte, Facebook, and everything else are.
And add Discord, Slack, and probably Telegram right next to ICQ.
And make sure this information is displayed immediately in the messages under the nickname, not just ICQ.
With this, the transition will be as smooth as possible within a year. When creating new tournaments, you can stop accepting entries without a new messenger, and things will quickly improve.