My mom kept a backup of her starter in a old coffee can in the freezer for like 15 years and honestly it saved her more than once. She'd forget to feed the jar on the counter during tax season (she did books for a few shops in town) and the frozen stuff always bounced back fine after a couple feeds. I never got into sourdough myself but I do the same thing with my grandma's biscuit recipe, got copies in my recipe box, my phone, and taped inside a cabinet door.