I think in later Uno revisions they added a IOREF pin for shields to read what voltage the IO is at and could in theory adapt to 5V or 3.3V as necessary. But y

I think in later Uno revisions they added a IOREF pin for shields to read what voltage the IO is at and could in theory adapt to 5V or 3.3V as necessary. But yeah it's still a pain in the butt for older shields that assume they're at 5V. Lots of sensors are happy at 3.3V though and many breakout boards are built to accept both 3.3V & 5V (with a level converter to take 5V down to 3.3V).