More than anything else, this is about changing conceptions and definitions of "in tune." The current conception of being "in tune" comes from the ubiquity of auto-tuning software that mechanically corrects a singer's note to exact correspondence with an expected pitch.
Traditionally, however, only highly trained classical singers would have been expected to stick to the pure, European-scale tones we now think of as "in tune." A more relaxed definition was stylistically appropriate for genres like rock, punk, folk or grunge --the inexactness of the pitch was a sign of authenticity.
Conversely, the skilled singers of other genres, like soul pioneer Ray Charles, or jazz legend Billie Holiday, deployed strategically detuned vocal notes and notes that slide in and out of pitch to convey complexities of emotion and nuance (similar to bending a note on an electric guitar).
Finally, to go yet a step deeper, the modern equal-tempered scale is itself artificial, and musical traditions like the blues characteristically deploy notes that are out of sync with the tempered scale as a way of evoking an older, less European, and more natural approach to music. In fact, it was a tribute to the seemingly out-of-tune singing of rural Black American folk singers that led the seminal blues popularizer W. C. Handy to write his all-time classic "St. Louis Blues".
Or, going back to European music from the pre-tempered era, and sometimes still heard in acapella choral singing, there's the possibility of the entire key drifting, due to the imperfect matches between the mathematical ratios of different pure tones.
If you're used to the older styles, none of these examples may sound out of tune to you. For listeners who have grown up on modern pop, however, it's likely a very different story. Is it possible that the pendulum has swung to its farthest extent, and that imperfect pitch will make another comeback? Or is this just an example of the increasing primacy of technology over even our aesthetics? Only time will tell.