Is that a backward compatible change in the sense of gRPC?

I think this is declaring something that is exported on the sourcegraph-frontend's grpc server. IE there is probably a corresponding PR on the sg repo to remove the endpoint. So this is safe. For the server side removal it will also be safe because no one is calling it.

I meant backward compatible in this sense

https://learn.microsoft.com/en-us/aspnet/core/grpc/versioning?view=aspnetcore-7.0#protocol-breaking-changes

Anyway, it will probably work ;-)

It is backwards incompatible in that sense, but it only really counts if a client is trying to call it.