I'd say Reddit and Discord are the two main competitors to forums these days. I didn't even know Slack had public servers, always thought it was more of a private thing.
It's tough, you really have to have a niche that your forum would cater to that isn't already being done. UX design seems like too broad of a topic, and r/UXDesign has 153k members in it currently.
However, if you provided something in addition to the forum alone, it would give your community a reason to congregate there. For example, my website jkhub.org is a forum but our main service is hosting mods. People mostly come to download mods, but it has had a fairly active forum community in addition to it for the last 11 years. Though since it is a gaming community, Discord has decimated our activity over the last few years. But it still has its purposes.
Forums need to still exist. The amount of information being gatekept behind these private sources like Discord, Slack, Telegram, Facebook groups, etc. is going to be the death of search engines as well. Majority of people search "reddit" in their search queries to get actual helpful info from real people rather than sponsored articles, and now Reddit itself is going to have an exclusive contract with the highest paying search engine meaning their content will be gatekept too.
Bottom line, yes, it does make sense, but it takes a lot of work to draw people into it and even more work to keep them around.
