Get stuck with expired key of a removed package
The Skype repository is a little wonky, as Microsoft has not updated the keys. Partly because Skype is eventually going away, and Teams is going to be the 'replacement'.
The only way to get rid of the warnings about the insecure repository is to remove the repository from your sources and then run apt update - it will no longer try and query the repository and as such you won't get your "NO_PUBKEY" or "EXP_SIG" notices.