I read that SSDs don't actually guarantee to keep your data if powered off for an extended period of time, so I actually still do my backup on HDDs. Someone please correct me if this is wrong.
Tape, M-DISC, microfilm, and etched quartz are the only modern mediums that are meant to be left in storage without needing to be babysit, in climate controlled warehousing at least.
(More relevant might be that backups are a largely sequential workload and HDDs are still marginally cheaper per TB than QLC flash.)