This is where the distinction between “an LLM” and “a user-facing system backed by an LLM” becomes important; the latter is often much more than a naive system for maintaining history and reprompting the LLM with added context from new user input, and could absolutely incorporate a step which (using the same LLM with different prompting or completely different tooling) edited the context before presenting it to the LLM to generate the response to the user. And such a system could, by that mechanism, “forget” selected context in the process.