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Search for IPv6 here https://dev-sec.io/baselines/linux/ this covers mostly ND and ICMPv6 related controls in Linux but easy to generalize
Is rp_filter enabled (kernel)
Are IPv6 bogons blocked on input and output (firewall)
https://team-cymru.com/community-services/bogon-reference/
Access controls - IPv6 needs proper firewalls configured as you can no longer rely on half-baked solutions like NAT for access controls.
Privacy extensions on clients
Specific ICMPv6 types to be allowed
If they use BGP, do they follow MANRS