In 2020, the pixel of mobile phone is so high now, is it necessary to buy Canon 40d?

If you only care about pixels. There's really no need to use the camera again.

Cameras, especially interchangeable-lens cameras, are not used in the same way as mobile phones. The camera may achieve wider shooting by changing the lens, such as ultra-wide scenery, shallow depth of field (blurring) of portraits, macro insects and weeds, and long-distance shooting of the moon and birds. Not to mention street sweepers looking for beautiful women.