So, I see that Servants still alive today are on the banned list. The Servant I have in mind can be interpreted as still being alive, but its very ambiguous and the land he refers to going to may be a metaphor for the afterlife. Since its ambiguous in his own legend may I choose to interpret it as him having passed on?
As long as you've got one version of the legend where they died, I'll allow it. Someone like Hanuman is banned because no legitimate Hindu source will say that he died on account of the boon with Yama, and someone like Merlin's banned because we know for a fact he's alive and well in canon. So yeah, as long as they're not definitively going to still be around by their mythos's logic, it's fine. I'd recommend PMing me about the particular individual just to be safe, though.