Quibble documentation still has a reference to the obsolete MW_COMPOSER_MERGE_MW_IN_VENDOR environment variable.
That was introduced in 2017 to merge composer development dependencies see for reference:
That should no more be needed since porting the logic to Quibble with: https://gerrit.wikimedia.org/r/c/integration/quibble/+/422108 and I could not find anything still referring to it beside CI config and a Quibble documentation.