As discussed at the AGM:
- Only allow formula/cask forks to be used when either blessed by
the source or used by two other major distributions,
- Further elaborate and tweak reasons to deprecate/disable formulae.
- Allow more versioned formulae when widely used.
- Add a document (based on above) for deprecating/disabling casks.
While we're here:
- let VSCode Markdown linting remove the trailing `!` from an
Acceptable Casks heading
