It depends on your definition of accessible. This is the tool kernel devs use, and that patch is intended for them. It's mostly a matter of getting used to it.
Github's user interface was scary at first for me, much more than a plain-text mail. So was GitLab.
Some people I know have complained to me that I send plain-text mails and that nobody wants to read a wall of text. I think that's probably a user issue, and it can in turn help filter low-effort comments.
If talking about a11y, plaintext mail ought to be one of the most accessible formats: high-contrast, can be processed by multiple tools including screen readers. ASCII art trough screenreaders is probably an exception.
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Now, about your specific question: there's patchwork[1], though I don't think that instance covers the general LKML.
