Add a S->C request/notification to update the clients config

Example

{
  // scope
  "myLsp": {
    "ignoredIdentifiers": [
       // existing content...
       "file"
     ]
  }
}

This would update the clients myLsp.ignoredIdentifiers configuration.

Use Case

Take the following example:

file = { statement* }
statement = { "hello" | "goodbye" ~ ";" }

Our LSP for Pest would report file as an unused rule, as it is only used from Rust code (which our server has no knowledge of). This is why we have the alwaysUsedRuleNames config, which suppresses that warning.

We want to provide a quick fix to add file to that list, but as far as I can tell there is no way for the server to request an update to the client's configuration file.

Is this against the principles of the LSP or just something that hasn't been implemented?

The issue names no files or tests. Start by reviewing the language server protocol's existing server-to-client requests and configuration-related capabilities, then determine whether a client-configuration update request or notification fits the protocol. Done means resolving whether the behavior is permitted and, if so, specifying the protocol change and its client/server semantics.