Forum software runs threaded discussion: boards, topics, replies, member profiles, moderation queues. The five applications here (phpBB, MyBB, Simple Machines Forum, Flarum and Vanilla) are self-hosted PHP and MySQL, which means the whole archive lives in a database you own and stays readable by search engines.
That last point is most of the reason forums never disappeared. Facebook Groups, Discord and Slack absorbed an enormous amount of community traffic, and none of them leaves anything behind. Fifteen years of answers inside a Discord server cannot be found on Google, cannot be exported into a knowledge base, and stop existing the day the platform changes its terms. A public forum is a permanent, indexable, ownable archive of questions and the answers people gave to them, which is why product support communities in particular kept using them.
The commercial alternatives are real and worth pricing. Discourse offers a limited free tier and charges $100 a month for Pro and $500 for Business; it is also free to self-host, though it is Ruby and Docker rather than PHP. Circle is $89 a month on Professional plus 2% on payments. Against those, phpBB or MyBB on a standard hosting plan costs the hosting and nothing else, with no per-member pricing and no ceiling on how many people register. All five install in one click from the My365i control panel.