At some point we need to rebuild CI containers with Composer 2.0 bundled, rather than Composer 1.x (I'm not sure if there's value in bundling both... Maybe there is for a while? For REL1_31 and maybe other usages?)
In https://gerrit.wikimedia.org/r/c/mediawiki/vendor/+/667051/2/composer.json I changed the composer-plugin-api version to ^2.0 to prevent (after it's merged) anyone accidentally running things with composer 1.x, but it won't pass CI.
Certainly, for now, it could be ^1.1||^2.0, but eventually, it seems a useful safeguard to have so that they get a strong warning if they try to run it with old composer 1.x
MediaWiki 1.31 doesn't support composer 2, and never will... And is EOL in June 2021, so we probably don't care about this use case too much. And we may just want to wait until that has passed, and drop composer 1.x support in CI completely.
At some point, https://blog.packagist.com/deprecating-composer-1-support/ will start to affect us
Reduced v1 metadata API update rate starting in May 2021
The update rate for new versions will be reduced from every-minute currently to once every 15 minutes. This means new releases will take a few minutes longer to be available for installation with Composer 1.x.
I know 15 minutes isn't a lot, but it's going to be enough in cases for CI's composer queries to fail for a little while...