Domain name forums are in decline, partly because of the loss of popularity of this format, but also because they were historically frequented by domainers, a population that is also declining.
Namepros.com is the leading Anglo-Saxon forum specialising in domain names. It mainly features sections for buying and selling domain names. Dnforum.com was historically the leader, but was abandoned, then relaunched in 2017 by a new team, without managing to recapture the success of the past.
Other key forums include:
Many webmaster forums have a well-developed 'domain names' section, such as WebmasterWorld, WebhostingTalk or DigitalPoint.
DomainCord is a Domain Investor Discord Community with insights and opportunities. English-language domain name Facebook groups are of no use whatsoever. They are frequented only by novice sellers who post offers for worthless domain names, often at unreasonable prices, and which therefore almost never sell. Half a dozen domaining forums are available in English.