On a typewriter, you'd be able to just adjust the carriage position to make a continuous dash or underline or what have you. Typically I see XXXX over words instead of strike-throughs for typewritten text meanwhile.

Most typefaces make consecutive underlines continuous by default. I've seen leading books on publishing, including iirc the Chicago Manual of Style, say to type two hypens and the typesetter will know to substitute an em-dash.