No one is certain how GW will handle the timeskip for T'au. Way I see it, GW can pick from Time shenanigans, stasus shenanigans, rename shenanigans, and ignoring the contradictions.
Ethereals maybe live for centuries, maybe not. The fluff isn't particularly clear, I don't believe. Firecaste T'au should mostly be dead from old age, if some of the older fluff remains consistent.
Farsight is protected against the ravages of time because of his sword. Shadowsun is eligible for freezing. Darkstrider and Longstrike, I would think, would not be so valuable as to be worth freezing and keeping out of the field for an indefinite period of time. Between engagements, sure, but frozen for the long term? Aun'va is dead, and is now a hologram or phased out. Aun'shi, last I heard, was missing in action somewhere in Commorragh's arenas.
Without models/rules, we've got the rest of the Eight, some of whom are biologically or cybernetically immortal, and some who are already receiving the Dread Pirate Roberts treatment. There's Angkor Prok whom we haven't heard from in at least a decade. How long do Kroot live? We don't know. From Warzone Damocles, there's Aun'Do, Sternshield, and Swiftflame, the latter two received a fluffing treatment in the recent T'au painting Guide. And we've got the Forge World characters, and probably a dozen other one-off characters I can't remember.
GW could easily just rename and Dread Pirate Roberts many of the character models, keeping general function, feel, and fluff intact, but somehow that potential loss of branding doesn't feel like GW (unless I look at AoS).
Someone mentioned Astra Militarum, but my understanding is that Human nobles have some sort of immortality juice they give out to people they like. I suppose T'au could get something like that for the vets, but I largely suspect it was just GW handwaving and ignoring the issue, and I largely suspect that's what'll happen for T'au.