Most IPV6 BGP configuration i can see both of these commands under router BGP
no bgp fast-external-fallover
no bgp default ipv4-unicast
From my understanding "no bgp default ipv4-unicast" is to make bgp advertise ipv4 and ipv6 at the same time if you are running dual stack.
For "no bgp fast-external-fallover" command. I do not know if this is necessary since you want BGp to session to go down immediately so that if you have iBGP with another router if will failover quickly.
fast-external-failover - on link flap, do not wait until keepalives are lost to shut down the peering.
You can disable it if you have temporary flaps on your connection to peer.
default ipv4-unicast - what should we do with IPv4 unicasts by default.
I've only seen this command used in IPv6 testing and on CCIE preparation materials :-)
Actually the 2nd command is widely used in SP environnment where you want your PE to advertise only VPNv4 address-family. This command means the PE will not advertise any address-family (by default it will announce IPv4 AF) and you have to specifically activate your neighbor under the AF you are interested in.
From what I understand disabling default ipv4-unicast - will enable you to configure other address families under the MBGP process, could be multicast, IPv4, IPv6, VPNv4, etc... it is not only used to enable or disable IPv6 capability under the MBGP process.