This is a work in progress answer. As always alternative answers are welcome.
The -mempoolfullrbf option when set to 1 ignores RBF signaling and assumes every transaction can be RBFed. This option was introduced in Bitcoin Core v24.0 although it was off by default in 24.0. For more discussion on the introduction of the mempoolfullrbf option see this question.
If the default mempool policy in Core was set to mempoolfullrbf=1 then one would assume that within 1-2 years the majority of Core nodes (and the majority of nodes on the network) would be running this policy (when they update to a new version). Of course users have the option to change from the default but in reality most seem to run with policy defaults.
The V3 Policy proposal introduces an additional set of rules to existing standardness and policy rules (V2 Policy?). This includes RBF signaling so it seems to be proceeding assuming that transactions do still need to signal that they are replaceable and we are not assuming a network of nodes running the mempoolfullrbf=1 option. Even if the majority of the network was running that option one would assume there would be a minority of the network checking RBF signaling rules were being applied correctly.
