Is Wi-Fi 6 backwards compatible with older Wi-Fi standards?

Yes, WiFi 6 (IEEE802.11ax) works with legacy standards as follows:

• IEEE802.11b — Wi-Fi 1 (2.4GHz)
• IEEE802.11a — Wi-Fi 2 (5GHz)
• IEEE802.11g — Wi-Fi 3 (2.4GHz)
• IEEE802.11n — Wi-Fi 4 (2.4GHz & 5GHz)
• IEEE802.11ac — Wi-Fi 5 (5GHz)