Does 20.3R3.8 support Q-in-Q tunneling?
Yes, on Juniper Networks QFX5110 Series QinQ is supported on version 20.3R3.8.Reference information below:Q-in-Q -- Configuring Q-in-Q tagging behavior for the native VLAN -- Junos OS 19.3R1†https://apps.juniper.net/feature-explorer/select-platform.html?typ=1&category=Switching&pid=31705110&platform=QFX5110&swName=Junos%20OS&rel=19.4R3&sid=1087&swtab=Junos%20OS