What year did it become impossible for a POTUS candidate to get a 50-state sweep?

[quesaisje]

Impossible is a high bar. Electoral landslides often occur under circumstances that no one could have predicted even a few years in advance. We shouldn't assume that the ossified electoral map over the past twenty years is a permanent feature of our politics.

I voted for 2004 in this poll, under the assumption that a reaction to a disaster like 9/11 could have produced at least a near-sweep for a Republican incumbent if the timing were right relative to the next presidential election (just to name the most imaginable scenario).