It is very far from a solution in 2019.
I've never had a problem with DCC transfers, but I suspect I'm in the minority.
"It does not keep history of discussions."
Actually, there were several servers which had "MsgServ" bot which would remember your last logout time, and could go back to retrieve all those messages from its logs and auto-relayed them to you, all you did was ask it to replay missed content from x channel and you got it.
"It does not have media embedding."
If you used pIRCh98 as your client you had live video capability and could easily do a quick desktop share through OBS or similar software, but that did rely upon everyone using pIRCh98.
Slack is basically a modernized version of a mix of the best features that IRC clients and daemons and bot creators had.
The only thing slack hasn't seemed to copy yet from all the old IRC stuff is the ability to use mIRC as a desktop file system browser (that was fun to see being coded.)
IRCd - real time, ram buffering of messages. Quassel - reads and stores buffers in a database, allows users to retrieve them.
If only there were some sort of Universal scheme for denoting the Location of a Resource. That would let you map a relatively short string to one of many possible file access services, and put that string in a text chat channel, rather than conflating two distinct problem domains.
Also every server I know and most clients do logging.
So dial back the snark.
How does IRC handle end-to-end encryption in groups?
How does IRC handle editing messages?
Servers can re-send history when a client join. At least InspIRCd supports this (though it's not enabled by default.)
Blowfish normally.
Press up arrow, retype message, or type *correction.
Which specific daemon did you settle on?
Not really looking for replacement though (team is currently on Mattermost)
When Atlassian finally pulled Hipchat last year I was tasked with finding the replacement. Turns out the one that met everybody's needs best was a local IRCd and whatever desktop client for it people liked.
Some problems have been solved correctly for a while; chat is one of them.