Here's a fairly large survey from TaleWorlds.
https://forums.taleworlds.com/index.php/topic,368157.msg8809762.html#msg8809762My understanding is that TeamSpeak is far more popular because of how clean and familiar it is for the people who use it. Discord could probably be way more popular among gaming clans if it A) had better voice chat quality in general, and B) allowed people to pay for high-priority/quality servers with more benefits, functionality, and UI customization, and have paid servers ignore that data-recording-for-ads thing that has turned some people away from using it. I'd use Discord for clan stuff if it had better quality voice chat, a cleaner interface that allowed people to have multiple displayed roles (like admin, rank, medals), and if more people were willing to use it. But I use it daily for non-clan stuff since it's a free option for small friend groups.