Comment (by Roland Haas):

Ah not quite. My fault though, sorry. I put an extra “s” into the redirect string. The trac ticket URLs are eg https://trac.einsteintoolkit.org/ticket/2135 so it should have been

RedirectMatch "^/ticket/([^/]*)" "https://bitbucket.org/einsteintoolkit/tickets/issues/$1"

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2240/redirect-from-old-trac-ticket-urls-to