Everyone in this comment section is yelling about how bad discord is, telling people to use forums or matrix instead. No one is asking "why?". Why aren't people using forums or matrix? Because the path to user growth isn't guilting people into the 'morally correct' choice, it's making a product they want to use.
Why are small communities using discord over forums? Well, we're talking about small projects, hobbies, and volunteer work. Hosting a forum costs both time and money - renting server space and configuring/managing both the forum and the server. Making a discord channel is instant and free. You want your favorite project to have a forum? Then take up the mantle of hosting and maintaining it yourself. You want all projects to use a forum? Develop a forum system that you absorb the hosting costs for. Neither of these exist, so communities use discord.
Why are small communities using discord over matrix? I'm in my 30s, I spend all day on my PC, I've taken a couple years of college courses in programming. Figuring out matrix was annoying for me. I had to figure out which client program to use, I had to navigate the less-than-ideal way of joining servers, and there was a difficulty curve for understanding the program's features and how to use it. It wasn't impossible, but it took effort. Discord doesn't. For every step of friction, a product will bleed users. Matrix is cumbersome to set up and use, and it's copying something that already exists and does it better for the end-user experience. It shouldn't be surprising that people prefer discord. Want that to change? Start contributing code to matrix and refine the user on-boarding process.
Instead of stating opinions, ask questions. That's how things get changed. No amount of moral grandstanding will change end-users, no matter how correct you might be.