Tried to objectively evaluate the options from the perspective of new players.
One huge advantage of Discord compared to other options (except Slack, but even with it, the process is a bit longer) is the speed at which a new player can start using it.
For Discord:
1) Click on the link
2) Enter a nickname
3) Enter an email and confirm it
4) Gain access to all player contacts and be able to send them messages directly, without having to search for contacts, add them, or install any clients.
What problems did I notice with ICQ when I tried to use it (I tried to enter my UIN and password, but they didn't work anymore, so I started from scratch):
1) You need to install a client (the official client already offers to enter a phone number right away), or you can log in using UIN, email, but only if it exists and works, and the most interesting thing is that now you can only register by phone:
https://icq.com/join/ru

2) I couldn't figure out how to add an email with a regular password so that I wouldn't have to use my phone every time in the program. Apparently, this needs to be done through the ICQ website, and I'm not sure a new user would want to deal with that.
3) And then the list of contacts is empty, now you need to add contacts, another step, I didn't check how this works now, I hope there are no problems with this, but nevertheless, this is another action.

The new official ICQ client, by the way, has now become a copy of Telegram in terms of appearance.
What problems does Telegram have:
1) Tied to a phone number.
2) You need to install a client, yes, there are web versions, but the connection to the phone is still significant, see the next point.
3) Adding contacts, to find and add a contact in Telegram, you need the phone number to be in the phone's address book, only then will it appear in the search. I have repeatedly encountered this problem when adding someone new if I didn't have their phone number. It was generally impossible to add a contact from the computer client until the phone was added to the address book.
Therefore, from the point of view of development, Discord simplifies the process for beginners.
ICQ is conservatism, yes, it works when you have a UIN and have been using it for 20 years and know every button, all contacts are added, and you don't need to do anything anymore except send messages. But for new users, this is again a connection to a phone and new lists of contacts.
Telegram is great and modern, but it has even more ties to the phone, which creates additional barriers.
--
In the context of attracting new players, the HotA community on Discord has over 1800 users, and HeroesWorld tournaments will now be announced there.
For new players from there, in order to participate, they won't need to do anything for communication, since they are already registered on Discord.
But if we have a mandatory different client, this could become a certain barrier, and we will simply be bypassed just because of the client, since this is an additional action, when the trend is that many want there to be one magic button and everything is simple, otherwise they won't use it.