Paper tickets on US domestic have been on the way out for several years. In fact all US airlines now charge you for issuing them in the first place. Works well most of the time except when the computer system(s) crash, then everything grinds to a halt.
If you lose/misplace/forget a (paper) ticket or e-ticket ref. number, you will be required to purchase a replacement. The airline may or may not charge a walk-up fare and/or a 're-issue fee' at their discretion. Unless you have mega status in their FF program, you will end up paying handsomely. If the original ticket does not resurface within 6 months you may apply for a refund of the original fare.
Or something like that.