There used to be a rule on Survival that stated (something along the lines of):
"No fleeing to non-pvp zones to avoid/survive pvp"
This was an issue in revisions 7 and before, when spawn was an just area of the map that could be easily re-entered just by walking back into it. People who were about to die would run back into the spawn zone where they couldn't be hurt.
I did not know when this rule was removed, but it no longer appears to be on nerd.nu/rules.
There is still (at least) one notable way to re-enter spawn, via The End.
This rule is being half-enforced. Mods who remember this rule are still enforcing it, telling people that they cannot stay in spawn if they are chased through the end. Newer mods, especially those who have come to Survival after being modded on PvE or Creative, do not enforce this rule, because it is not on the rule page.
I think we need a definitive answer from the Survival Admins about whether this is a rule or not.