I think some information about esp8684 was already published on Chinese websites (e.g.
https://view.inews.qq.com/a/20211223A0AUX400), but it hasn't reached the official Espressif website yet. The brief summary looks like this:
ESP8685 has an ESP32-C3 silicon die packaged together with 2MB flash in a 4x4 mm QFN-28 package. This package doesn't have SPI Flash signals bonded out to package pads.
For comparison, ESP32-C3F2 also has an ESP32-C3 die and 2MB flash, but the package is 5x5mm QFN-32, and SPI Flash signals are bonded out.
For software developers, ESP32-C3F2 and ESP8685 are equivalent, you can use "esp32c3" target in ESP-IDF to compile software for them.
ESP8684 has an ESP32-C2 silicon die packaged together with (some amount of) flash in a 4x4 mm QFN-28 package. Just like the ESP8685, this package doesn't have SPI flash signals bonded out. ESP32-C2 is a lower-end chip compared to ESP32-C3, with less RAM and peripherals. Support for this chip is being added to ESP-IDF at the moment. I can't say yet whether ESP32-C2 will be marketed and sold only as part of ESP8684 package or separately as well.