QinQ ethertype tags changed
In 2.7/23.01 QinQ uses if_vlan instead of netgraph and set the outer tag ethertype as 802.1ad (0x88a8). That is the expected tags tag type for most QinQ links but is not what netgraph set in 2.6/22.05 and earlier. That used 802.1q (0x8100) for both tags. That could lead to existing QinQ links failing at upgrade.
This change needs to be documented. The new behaviour is probably correct for most new interfaces.
There should be an option to revert to the previous behaviour if required. ifconfig can set the vlanproto value directly in 23.01.
Existing QinQ interfaces could be automatically set to that at upgrade to maintain compatibility with connected devices.