Sorry for not being clear. When the signal does not repeat, the Nyquist limit isn't a factor because there is no possibility of moire. That's where the difference between sharpness and resolution comes from...single points don't have frequencies. Between the Nyquist limit and the sensor resolution, there are no guarantees about how or whether a feature will or won't resolve and the photographer has to use their judgment relative to the image's purpose. An anti-alias filter takes the necessity of such decision off-the photographer's plate in exchange for limiting the absolute possibilities.
Astro-photography is an illustrative example. Celestial objects don't appear in night sky scenes with a regular frequency. So there is no need for an anti-aliasing filter. It just lowers point resolution without any benefit. So cameras for astro-photography often lack AA filters.
The limit remains a factor for non-repeating signals. You get speckle. It shows up in natural scenes like leaves against the sky, fur, grass, sand, etc.
Astro-photography is an illustrative example. Celestial objects don't appear in night sky scenes with a regular frequency. So there is no need for an anti-aliasing filter. It just lowers point resolution without any benefit. So cameras for astro-photography often lack AA filters.