I have installed packages 6to4 and luci-proto-ipv6. However, in LuCI's drop-down for interface protocols, I only see the usual DHCP, DHCPv6, Unmanaged and Static.
Why do you want to use 6to4 anyway? The anycast relay address is deprecated. And without that I guess 6to4 is only useful between ipv4 hosts, or in controlled environments.
Actually I don't have native IPv6 at home. I use 6in4 tunnels from Hurricane Electric.
There are similarities between 6to4 and (plain) 6in4, they use the same protocol number (41) and the payloads also both contains IPv6 packets. But 6to4 used an anycast prefix as default gateway to reach IPv6. With 6in4 you need an agreement with a 6in4 tunnel service such as Hurricane Electric.
Due to unsolvable operational problems using the 6to4 anycast prefix, that part of the standard was deprecated in 2015.
Without the anycast prefix 6to4 isn't very useful.
To answer the actual original question - a reboot or a network restart (not reload) is required for netifd to pick up newly installed protocols (which are then queried by LuCI)
Thanks, that fixed it. Meanwhile my question has become doubly redundant because I found I'm behind CGNAT where 6to4 doesn't help, and the server which only reported an IPv6 address last week now has an IPv4.