Unknown Per the documentation:
Matrix Variant Definition
The matrix syntax is deprecated in favor of the generate.tasks command. Evergreen is unlikely to do further development on matrix variant definitions. The documentation is here for completeness, but please do not add new matrix variant definitions. It is typically incorrect to test a matrix, as a subset of the tasks is usually sufficient, e.g., all tasks one one variant, and a small subset of tasks on other variants.
All of our evergreen projects (driver, mongoid, bson, etc.) make heavy use of the matrix variant syntax. It's probably worth spending some time to migrate to the favored syntax, to avoid our test infrastructure growing too stale/unsupported.