Today I learned, adding tabindex="-1" to skip link targets is likely no longer necessary in modern browsers.
Manuel Matuzović tested skip links without tabindex="-1" on targets across major browsers and screen readers on macOS, Windows and Android, concluding it is safe to remove in most setups, thanks to sequential focus navigation starting point.
Furthermore, adding tabindex="-1" to skip link targets also introduces a regression in usability. With tabindex="-1" on main, clicking on any non-interactive area inside main makes main itself the focus starting point. So, the next tab press sends focus to the first focusable element in main, not the next one after where the user clicked. The gov.uk team removed tabindex from main for this reason.