The change streams test suites all utilize the set_read_and_write_concerns.js override as a means to override write concern to "majority" on all commands run in their tests. This was presumably added originally to make sure that writes done in a test would be majority committed so as to ensure they would be visible in subsequent change stream queries (which only return majority committed data). Hopefully the reliance on "majority" write concern has been superseded by sufficient usage of assert.soon to wait for changes to appear in a stream. Getting rid of these overrides would be a nice cleanup and also make it more explicit when tests do and don't rely on majority write guarantees.