In the Retryable Writes spec, it is stated that an error is retryable if it is "a server error response without an error code or one different from those listed above, but with an error message containing the substring 'not master' or 'node is recovering'".

It is unclear whether newer versions of the server still return error messages like these without the appropriate error codes. If not, this can be clarified in the Retryable Writes spec. If so, perhaps these errors can be assigned the proper error codes/error labels. Either way, the drivers can stop parsing error message strings as older versions of MongoDB are deprecated.