Do all devices with an A8 chip support iOS 13 and iPadOS 13?

No, the iPhone 6 was the first A8 device and it is not compatible with iOS 13. iPhone 6 can only be updated to iOS 12.4.4. I assume that’s a limitation of the 1GB RAM in the iPhone 6 and 6+. The iPad mini 4, which also uses the A8, has 2GB RAM.

This is the list of supported devices:--> iOS 13 and iPadOS are compatible with these devices - Apple Support

Regards

Giulio