The "threat model" of OP is the consequence of a wrong usage pattern. You keep your identities separate. If you're in a high-stakes environment where being DDoSed or attacked is an actual threat, you compartmentalize that identity adequately by going through a proxy, VPN, TOR, I2P, or using a different medium altogether. Fortunately, unlike other services, this is extremely easy with IRC since the identity is just a nick, and nothing prevents one from having as many identities as needed.
This obsession with having a single centralized identity where some vendor is trusted to painstakingly guard the linked PIID is misguided, unsafe, and harmful. Discord will fall prey to a data exfiltration attack eventually, and affected people will only then realize that this trust was misplaced. The fact that people are comfortable giving Discord their phone numbers while being worried about their client IP being exposed on IRC is baffling.
It's not a matter of one single identity. Whether you use one or multiple you have a real practical problem which does actually happen which is prevented by discord without the need for using a 3rd party tool or you have a selection of alternatives which don't. Giving discord your phone number (which theoretically could be a problem) is really not an issue by comparison (hint: while not exactly the paragon of virtue, discord in general is more trustworthy than a random user of it).
It's not a real practical problem – and I'm not sure why we're pretending that it is one. Let's clearly state what the problem is: OP wants to play MMOs & online games against potential threat actors who may DDoS and/or doxx them. Which is why they simultaneously also want to shield their identity from them. An analogy would be somebody who wants to play games with the neighbourhood meth gang while not wanting to get stabbed.
This is not safe or reasonable behaviour. Even if these people lack a client IP, a motivated attacker can piece your identity together from what you say and/or post eventually. Discord just makes that a bit more difficult at the cost of you handing over PIID of greater importance.
Then don't talk to them on a public Mumble/Ventrilo/IRC server without adequate precautions.
Most MMOs & online games offer an in-game chat client that does appropriate cloaking & is also moderated (so an offender risks a ban if they engage in harassment). Nobody is forcing OP to invite these people onto his VoIP server.
This obsession with having a single centralized identity where some vendor is trusted to painstakingly guard the linked PIID is misguided, unsafe, and harmful. Discord will fall prey to a data exfiltration attack eventually, and affected people will only then realize that this trust was misplaced. The fact that people are comfortable giving Discord their phone numbers while being worried about their client IP being exposed on IRC is baffling.