Is there another oil plug on a 1998 Harley Davidson Heritage soft tail classic springer besides the plug at the end of the drain hose?

No, that's the only drain for the motor oil. But if you're concerned about any dirty oil remaining in the motor when doing an oil change, you can do what's called an "extra quart" oil change.

You change your oil as usual but before starting the bike, disconnect the oil return line going back to the oil tank and add a length of the same size hose to it (about a foot or so) using a piece of 3/8" metal tube as a coupling (it will just slip on).

Put the other end in your drain pan and start the bike. At first the oil coming out will be black, but when it starts coming out clean (about a quart) shut the bike off, reattach the oil line and top off the tank.

This is an old timers' trick, and it will add many miles to your motor.