Take a look at https://github.com/42wim/matterbridge which bridges a lot of protocols.

It only support public channels at the moment, but we're planning to add user/user communication too.

this is what IRC ... does. The xmpp standard and ecosystem of clients/servers has also provided this for a decade plus.

If something 'modern' is required, Matrix protocol and its ecosystem is a contender. Also look into the world of ActivityPub federated applications, which cover everything from replacing Twitter to blogging.

My personal setup is to use bitlbee or libpurple to connect to every major chat network that exists today, bridged to my irc client via znc. This provides a consistent presence, status, and ui across desktop/mobile, push notifications, filtering, and behavior under my control, all with chat client that doesn't take up 1GB+ of memory with phone-home telemetry and useless emoji

sounds like matrix.org

I think bridges are not a good enough level of integration. You really need to have your personal connection to your IRC servers.

(Unless the matrix IRC bridge connects to the IRC server with a server-to-server type connection and can thus address messages from/to virtual users on the IRC protocol level. But that would need a lot of coordination and authorizations from IRC server administrators, which brings in so much friction that it probably kills the idea.)

edit: it seems possible that the matrix irc bridge will make a new client irc connection per bridged user, which sounds like a better idea. hard to make out from the docs though.

This is precisely what happens. The matrix.org bridge has approximately 25k open client connections to freenode. It would probably be a good idea to switch it to server-server connections to save resources, but afaict nobody has experimented with that yet. If you've ever seen users with an [m] at the end of their names, that's the default name for bridged matrix users.