Yeah. Just to be exact- we have a 2014 27"iMac (late 2014) with the "base" processor, i.e. Intel 4690. It is running Big Sur 11.6 and Safari 14.1.2 (16611.3.10.1.6) It behaves in the "good old way," that is, when you open bookmarks in the sidebar, then close Safari, then open Safari again, the bookmarks are displayed as opened, that is, you don't have to *reopen* bookmarks each time you open Safari.
We also have a 2017 27"iMac with the "middle speed" processor, i.e. Intel 7600. It is running Big Sur and Safari 15.0 (16612.1.29.41.4 16612) On this machine, I have the same problem you do, i.e. each time I open Safari I have to "show" the bookmarks sidebar, either by "clicking" on "bookmarks" or by shift-command-1
When I have some time to waste, I will create a bootable image, then "update" to Safari 15.0 on this 2014 machine and see if the problem appears. If it does, I will then restore from the bootable image. This is the only way I know to "downgrade" to Safari 14. My *guess* is that Safari 15 on this machine will run just fine, with the proper bookmarks presentation.
From the various comments from people on this forum, it *appears* that there are people out there running Safari 15.0 on Big Sur 11.6 who do *not* experience this problem. If that is the case, then different hardware sets react to Safari 15.0 on Big Sur in different ways. It is also possible that people think they are on Safari 15 when they are not. This is unlikely, I guess.
Right now, my bet is on hardware-dependent code in Safari 15.0
When I run my experiment, I'll let you know.