As this example shows, it's now standard for PG commit log messages to include a link to the relevant email thread, so all you need is the message-ID of the first message in the thread to search the commit log with.
Cross-posting my comment from Hackers Discord:
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The specific request here seems fairly doable as something we could offer as a custom search mode on the mailing list website. For a given time period show me all messages that began a new message thread where there are zero replies to the message. Apply to the -bugs list and you have at least most of what is being asked for. [...]