(note for me cause I will forget this)

ZNC allows you to have channel "playback" when you reconnect with your client, but it's easy to lose track of mentions, or not be able to see a mention on an active channel that surpasses 500 messages (ZNC max buffer) while you're away. For this you need to enable a ZNC module.

First, navigate to the ZNC user, then the network and scroll down to the module "watch" and enable it. (web admin url: /mods/global/webadmin/editnetwork?user=trev&network=libera).

Go into your IRC client and enter:

/msg *watch add * *mentions *%nick%*

Every time your nick is mentioned, it will add that message to a buffer called *mentions.

Next, we need this buffer to persist in ZNC. Navigate to your ZNC user (/mods/global/webadmin/edituser?user=trev) and find the Flags sectiobn. Uncheck "Auto Clear Query Buffer".

Now, when someone mentions you while you're away, once you log in again you will see these mentions in a nice little buffer.

Another caveat is that you must enable the "clearbufferonmsg" module or else the query buffers (like a private message) will stay open and load when you connect. If your client doesn't acquire the znc.in/self-message capability then all your messages will disappear from the query buffer, making the playback of a 1:1 message pointless.

EDIT:

I found an issue on ZNC.el that provides a solution to the above caveat.

Simply adding this advice to your erc config does the trick:

(advice-add #'erc-login 
            :before (lambda ()
                      (erc-server-send "CAP REQ :znc.in/self-message")
                      (erc-server-send "CAP END")))

Now query buffers (direct messages) can be restored and your messages will be present.