What would be the consequence if the Earth's axis tilt resulted in an obliquity of 0 degrees?

The Sun would always appear along the celestial equator with constant day length.

If the obliquity of the ecliptic were zero degrees, it would imply that the Earth's axis of rotation is perfectly perpendicular (not tilted) relative to its orbital plane. In this hypothetical situation, the Sun's apparent path, the ecliptic, would merge exactly with the celestial equator. Consequently, the Sun's apparent position would never shift relative to the celestial equator throughout the year; its declination would remain fixed at zero degrees. This lack of annual variation in solar altitude means that every location on Earth would experience a constant day length—always equal to the time it takes the Sun to cross the meridian—and thus, there would be no distinct seasonal variations caused by changes in solar energy intensity or day duration.