I ride in rural areas, am the only licensed driver in my home, and we do not have a car, so I have certainly thought about this (and, on rare occasions, used a backup plan.)

Most of my rides take me out of the Uber/Lyft service area and out of cell range. Once, I straight up hitched a ride; once, I was offered (and accepted) a ride from a friendly farmer; and once, I received some assistance from a couple of cyclists (one of my tools had broken!) Another option, which I have not used, would be stopping at a farmhouse and asking to use the landline to try and dial a cab - though hitching would probably be faster and easier.

My best backup plan is to prevent getting stranded in the first place. Making sure that my bike is in good shape, packing appropriate tools, and learning simple skills like how to boot a tire, change a brake cable, or rig a broken derailleur/cable/chain all go a long way to making it home without assistance.