Question

Suppose that Earth’s axis were tilted 90 degrees instead of 23.5 degrees. What might the seasons be like?

Answer 1

Wow !
Extreme extreme seasonal temperatures, and totally whacko lengths of day and night.
Every place on Earth would have something like this: ==> The length of daylight would start at zero, slowly increase to 24 hours over a period of 6 months, then steadily decrease to zero over the next 6 months. Of course, any part of the 24 hours that's not daylight would be darkness. ==> Average daily temperatures would track the length of daylight, from frigid to blazing and back to frigid over the course of 365 days. ==> The ONLY places that would have a solid 6 months of daylight, followed by 6 solid months of dark, would be the poles.

Answer 2

The seasons basically wouldn't change, because basically only one spot would be facing directly at the sun the whole time, so it would be summer there forever, and the edges that barely get any sun would be in eternal winter.