Currently, there isn't a feature to 'freeze' a gate in the way described. When a feature gate's status is changed to 'Launched' or 'Disabled', it will always return the default value (TRUE for Launched, FALSE for Disabled) for all users, regardless of whether they are new or old. The gate will stop generating billable exposure events, and you'll stop incurring costs. The gate reference is likely safe to be cleaned up in the codebase.
Implementing a feature to remember the previous evaluation for a gate would be challenging. However, if you want this sort of behavior, you could implement it yourself. If you have a client-side app, it would be fairly easy to store someone's evaluations in local storage, and check for a value there before checking Statsig.
For server-side implementation, you would need a database and a bit more work. This would allow you to store the evaluations and check for a value before checking Statsig, effectively creating a 'freeze' behavior.
Please note that these are workarounds and not built-in features of Statsig. If you have further questions or need assistance with a specific use case, please reach out for further assistance.