>In a recent message, another message was referenced like this:
>
>Message ID: nnnnnnn
>
>There was no clue who it was from or to, or anything about what that message said.
>
>ISTR it used to be possible to search on a message or thread number, but I couldn't find any way to get to that message.
>
>Eventually, another member posted a message that created an auto-link, like Message#nnnnnnn
>
>Is that now the only way to get directly to a message or thread?

As Naomi mentioned, simply use that ID in the message ID field in the top left toolbar. In there, you can enter either a thread ID or a message ID.