In extractWriteConcern this comment states that w:0 is interpreted identically as w:1.

However, the place where it is supposed to be converted to w:1 never actually gets run because it is within an if (!clientSuppliedWriteConcern) statement, and w:0 is a clientSuppliedWriteConcern. There does not seem to be any other place in the codebase where w:0 would be updated to w:1. Additionally, it seems like there are some differences in the server/shell/tests in how w:0 and w:1 are treated (see scripting/mozjs/mongo.cpp, write_commands.cpp, crud_api.js, repl_set_config_validators.h, read_write_concern_defaults.cpp).

I'm not sure if this is a bug and we want to actually change w:0 to w:1, in which case this should be moved outside of the if statement, or if we do actually want to keep w:0 as a separate thing that is not interpreted identically to w:1, in which case the comment should be updated.