It's also easier for a (technical) user to reference they got error 1018 than copy & paste "An index signature parameter cannot have an accessibility modifier." Especially in titles and when referencing it multiple times within a body of text.
How does an error code make it easier to look up an error? The workflow is either “get error, google it”, or “get errorcode google it”. In both cases the docs will be the top hit.
If we where talking 20 years ago I might agree, but I really can’t see the argument with todays tooling.
Updates can improve on the error messages, making them more clear, the error code does not change but the message changes.
Also many times the error message is interpolated with your specific details like
"Syntax error at line 123 in file /home/user/myfile , symbol X is not allowed here" this is a silly example but often enough when i google this errors I have to first strip out my data from them.
It also makes it easier for developers look up a specific error in the documentation, assuming it's been documented.