Besides only a few devs still use the IRC, so It’s possible that a discord server might be a good solution considering we’re closing into getting more “community-friendly” over the next months.
I think you are worrying too much here. Discord might be the hot stuff right now, but you said yourself that you didn’t really use it up to now, and neither have I or any other person I know face-to-face. As immense as the hype is, in the end Discord is still the new kid on the block, supported by a vocal minority, and it’s nowhere near as popular as the original gangsters like IRC or TeamSpeak/Mumble. Besides, we also have webchat for IRC and I’d argue it’s even easier to use than Discord since it doesn’t require registration.
Personally, I am not going to touch Discord again anytime soon. It only runs in browsers (and yes, the client is technologically a kind of browser, too), and in my experience browsers are notoriously recourse-hungry. It’s also an extremely closed system that doesn’t even allow running a server on your own system, while Pencil2D is an Open Source project using one of the strongest commonly used copyleft licenses around. When I want to have a text chat with other devs I use IRC. When I want to have a voice chat with my pals I use my Mumble server. We are used to it and it works very well for us, so we won’t move away just because there’s this shiny new thing.
tl;dr I think IRC and maybe TS/Mumble are currently better options, since they’re much more well-established than the hype around Discord makes it seem and also since they are more open systems. If some want to use Discord that’s fine by me but I won’t tag along and I don’t think it should be the primary means of real-time communication for an Open Source project.