Update 2021-03-28: We got feedback from MELPA: the modus-themes will stay there after all! The newer parts of the discussion start from here: https://github.com/melpa/melpa/issues/7245#issuecomment-808810507.

As you may know, the Modus themes are part of Emacs28 for a few months now. Their latest version was synced upstream recently (read the release notes of version 1.2.0). The upgrade took longer than usual because there was a major refactoring between versions 0.13.0 and 1.0.0: in short, the themes are no longer standalone files but are instead derived from a common source, share customisation options and functions/commands, and are easier to maintain.

This update sets in motion a series of events that I wish to be transparent about. Users updating from older versions to >= 1.0.0, are advised to read the announcement on the emacs-devel mailing list. The web page of the change log is also available.

Changes for GNU ELPA

GNU ELPA, the official package archive that comes preconfigured with Emacs, is instructed to build the new modus-themes package directly from upstream Emacs. It is considered a :core package, meaning that whenever we update the themes in emacs.git we will automatically get a new version on GNU ELPA. This is meant as a convenience for users who are running a stable version of Emacs, such as Emacs27.
GNU ELPA also serves as a point of reference and credible source provider for other package archives such as Guix (see issue 111 on the Modus themes’ issue tracker).
The old GNU ELPA packages modus-operandi-theme, modus-vivendi-theme (at version 0.12.0) will soon be replaced with code that automatically pulls in the new modus-themes package. Then, after a grace period of, say, 6 months, the old packages will be deleted and only modus-themes will remain. This has been discussed in Emacs bug#46964. I still have not had the time to study how those in-place substitutes will be implemented, but rest assured that it will be done.
The modus-themes in emacs.git and, by extension, in elpa.git are updated whenever I tag a new release. This happens every month or so. The next version is 1.3.0, which will be published some time in mid-April.

Removal from MELPA

MELPA maintainers, who have become aware of the status of the themes as described above, have asked me to delete the modus-themes package from their archive (see issues #7245 and #7246). The idea is that too many package variants can be confusing to users.
I will comply with the MELPA maintainers’ request to delete the modus-themes package from their archive. This will be done with a pull request against melpa.git either before the end of this week or the beginning of the next.
Existing users should be able to switch to the package provided by GNU ELPA. I honestly do not know whether the transition will be seamless (e.g. if you have (use-package modus-themes ...) or whether it will require manual intervention. My apologies for whatever inconvenience.

All of the above combined mean that there will no longer be a package archive that contains a version of the themes corresponding to the latest commit on modus-themes.git. To run the bleeding edge, you must either use something like quelpa or straight.el, or manually clone and maintain a local copy of modus-themes.git.

This is all for now. Please contact me if you have any questions. I am also posting this announcement on the Modus themes’ issue tracker, in case you want to comment there: https://gitlab.com/protesilaos/modus-themes/-/issues/167.