[mbed-tls] Status of RFC 8449 support

I saw the following comment when configuring Record Size Limit Extension (RFC 8449) in `mbedtls_config.h` [1]:

This extension is currently in development and must NOT be used except for testing purposes.

Is this still accurate? What functionality is missing for full RFC 8449 support? Is this feature planned for a specific date?

[1] https://github.com/Mbed-TLS/mbedtls/blob/611f899c0c9d397baedfaec34ea0861ad25...