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.
A disk that is powered off is not holding your data, regardless of whether it is an HDD, SDD, or if it is in redundant RAID or not. Disks are fundamentally a disposable medium. If you don't have them powered on, you have no way to monitor for failures and replace a drive if something goes wrong - it will just disappear someday without you noticing.
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.
Do you poweroff your backup HDDs for extended periods of time (months+)? That's a relatively infrequent backup interval. If not, the poweroff issue isn't relevant to you.
(More relevant might be that backups are a largely sequential workload and HDDs are still marginally cheaper per TB than QLC flash.)
