Every Plugin Hub plugin that talks to a server outside Jagex or RuneLite carries that warning automatically. It isn't about anything Kill Clog collects. When you look up a player, the plugin asks TempleOSRS and RuneProfile for their collection log, and like any request on the internet, the server you ask can see the IP it came from. That's the entire story. Kill Clog doesn't log your IP, doesn't store it, and has nothing to send it to. Jagex's own hiscores don't trigger the warning because RuneLite treats Jagex servers as first party.
Your synced log is a single file on your own computer, in .runelite/kill-clog, named after your character. Close RuneLite, delete that file, then log into the main game, open your collection log, and sync again. Clean log in about a minute.
No. Syncing reads your log from the game and saves it to a file on your machine so the panel can show it instantly. Nothing is sent to killclog.com or anywhere else. Public lookups work the other direction: they read what players have already chosen to share on TempleOSRS or RuneProfile.
Kill Clog can only show what a player has shared publicly. If your friend has never synced their log with TempleOSRS or RuneProfile, there's nothing to read. Their hiscores side still works, since that comes straight from Jagex. Once they sync with either service, their log shows up like anyone else's.
Four. !kclog <boss> posts your kill count and log progress for that boss, and shorthand works (!kclog cg, !kclog zammy). !missing <boss> lists what that boss still owes you. !3a and !gilded show your treasure trail rares. They work in public, friends, and clan chat.