On Tahoe, there is a simple menu to revert to a sidebar view you are used to. But if you cannot (or don’t want to) upgrade to Tahoe, you do have a couple options.
the first option, you can lay a new sequoia install over the old one without completely erasing and reinstalling sequoia. afterwards, you will have sequoia with safari 18.6.
and the other option, (the one I landed on after trying both options) you may want to look at 6x6's reply in this discussion. if you are comfortable using terminal, their suggestion fixes things for those of us on sequoia with safari 26.1.
please note, if you are going to try the terminal commands, you need to grant terminal full disk access. go to System Settings > Privacy & Security > Full Disk Access to do so.