Always? No. At some point in the past, yes.

definitely post-2012 (UA/CO merger), it has always been 24 hours after ticket purchases.

back in the days pre-merger on UA, and sometime prior to that, there were the days when you could hold a fare (for no fee - they monitized that now) and that hold was available until midnight the next day. So you could book a ticket at 12:05am Monday, and you had until 11:59pm Wed to purchase. There was even a time when that hold policy was in effect, and you could cancel 24 hours after purchase, giving you in effect almost 3 full days if you booked at the right time to change your mind without losing $. Those days are long past - this was probably at least 12 or 13 years ago. Now, no free hold, and the deadline to cancel for a full refund is 24 hours.

I haven’t seen a lot of reports since pandemic started, but UA was still at least pre-pandemic going above and beyond DOT policy on 24 hour cancellation by allowing this to happen no matter when the flight you book is scheduled (DOT requirement ends if the flight you book is departing within 7 days).

you could also be thinking of DL. Again, don’t know their current policy, but at least pre-pandemic, their cancellation for a full refund was available until midnight the next day instead of 24 hours.