Change 636227 had a related patch set uploaded (by Reedy; owner: Reedy):
[mediawiki/core@master] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/636227

Change 636228 had a related patch set uploaded (by Reedy; owner: Reedy):
[mediawiki/vendor@master] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/636228

With the above patch... It's not pretty, but is it sufficient/a good idea?

The "wikimedia/composer-merge-plugin" plugin was skipped because it requires a Plugin API version ("^1.0") that does not match your Composer installation ("2.0.0"). You may need to run composer update with the "--no-plugins" option.
> ComposerHookHandler::onPreUpdate
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires wikimedia/composer-merge-plugin 1.4.1 -> satisfiable by wikimedia/composer-merge-plugin[v1.4.1].
    - wikimedia/composer-merge-plugin v1.4.1 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.

You are using Composer 2, which some of your plugins seem to be incompatible with. Make sure you update your plugins or report a plugin-issue to ask them to support Composer 2.

reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/core$ git diff
diff --git a/composer.json b/composer.json
index 36284b40e0..d03a9caee0 100644
--- a/composer.json
+++ b/composer.json
@@ -18,6 +18,7 @@
        },
        "prefer-stable": true,
        "require": {
+               "composer-plugin-api": "^1.1",
                "composer/semver": "1.5.1",
                "cssjanus/cssjanus": "1.3.0",
                "ext-ctype": "*",

Change 636228 merged by jenkins-bot:
[mediawiki/vendor@master] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/636228

Do we want to do this "properly" or will this suffice?

Change 640495 had a related patch set uploaded (by Reedy; owner: Reedy):
[mediawiki/core@REL1_35] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/640495

Change 640496 had a related patch set uploaded (by Reedy; owner: Reedy):
[mediawiki/core@REL1_31] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/640496

Change 636227 merged by jenkins-bot:
[mediawiki/core@master] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/636227

Change 640496 merged by jenkins-bot:
[mediawiki/core@REL1_31] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/640496

Change 640495 merged by jenkins-bot:
[mediawiki/core@REL1_35] composer.json: add requirement for composer-plugin-api ^1.1

https://gerrit.wikimedia.org/r/640495

In T266418#6617235, @Jdforrester-WMF wrote:

Do we want to do this "properly" or will this suffice?

Any opinions? (Reedy maybe?)

In T266418#6617235, @Jdforrester-WMF wrote:

Do we want to do this "properly" or will this suffice?

Not sure. I wish I could say we could solve this with Documentation alone, but I doubt we can.

Especially when composer 2.0 is the stable/"default" download.

Probably worth mulling over a bit more