True or false the diaphysis of a long bone is considered the shaft that makes up most of the bone

It is true. Diaphysis is the shaft of the bone.

Explanation:

The diaphysis is the body or the middle part of the long bones. The body of the long bones is cylindrical form and it is built from compact bone tissue.

The body of long bones is formed by the calcification or ossification of the primary centers, as opposed to the bone ends formed by the ossification of the secondary centers of calcification.