In Sequoia 15.7.1 Safari sidebar changed and I'd like to revert to old appearance. How?

You did not say in what way the sidebar changed, but you can customize the display of items there.

Apple provides this guidance: Customize the Finder sidebar on Mac - Apple Support

If this is simply a matter of some items not appearing to be listed, check to make sure that the little arrow (caret), found to the right of each sidebar heading when you hover the cursor there, is turned down to reveal those items. If it is not turned down, then click it.