Has this ever happened to you?
You:
git pull
Your YubiKey: starts blinking to indicate that you should touch it to confirm that SSH should be allowed to authenticate to the git server
You: waits for git pull to do its thing
Your YubiKey: times out
git: fails
You: is annoyed
Have no fear, ssh-agent-notifier is here!
Run ssh-agent-notifier --target unix://$SSH_AUTH_SOCK --host $XDG_RUNTIME_DIR/notifying-agent.sock, then set SSH_AUTH_SOCK to $XDG_RUNTIME_DIR/notifying-agent.sock for everything else, and it will notify you whenever something tries to use any keys from your agent!
No warranty, et cetera. This may eat your cats or cause other trouble. In particular, I just ignore errors on notification sending, which means you may not actually get the promised notification and the software won't give a damn. But it works on my computer!