The bug

@p and @r can target dead players when they previously could not. As well, the distance/dx/dy/dz parameters should also prevent the @a selector from targeting dead players, but do not. Via MC-46223:

Searge added a comment - 05/Feb/14 10:53 PM
The @a selector (but only without a radius/area defined via r/rx/ry/rz) will include dead players in the next snapshot. All other selectors work as intended.

How to reproduce

Place the following commands inside a repeating command block chain. They will state your name while alive and when dead, except for @e which correctly excludes dead players:

say P: @p
say R: @r
say A: @a[distance=..20]
say E: @e[type=minecraft:player]