Is there a way to install old version and not update it?

Yes. See https://www.reddit.com/r/operabrowser/wiki/opera/rollback_version. You would need a build before 84.0.4295.0. But, you'd have to try to find one that's stable and you obviously wouldn't get any fixes that happened after the build you choose. You should also back up your profile folder just in case there were changes made to the profile format that are no longer compatible with the older build.

I would try a bookmark manager extension. See https://addons.opera.com/en/search/?query=bookmarks. Maybe one does things just the way you want.

It seems I'm doomed. Some of my folders can reach to hundreds bookmarks which I have to check, work and remove when I finished working with that page. Now I have to scroll down to the bottom everytime I open a week old bookmark. And I just checked a single folder it took 24 seconds to reach to the bottom. I was using FIFO (First In, First Out) management system on my works. So when I check an old bookmark and finish my work with it, I was removing it and chronologically newer one was getting close to the top. "Last In, First Out" doesn't work for my needs.

Is there a way to install old version and not update it?

The change was made because adding to a folder with the heart icon in the address field added to the top of the folder and the drag URL method was inconsistent with that. Opera always adds to the top with other methods.

But, to answer your question, there's no option or workaround to change it back. There's just the way of continuing to drag till you get to the end of the list like you said you don't want to do and are not used to doing.

Here I click and drag the url or bookmark onto the bookmark folder icon. But it puts the new bookmark to the top of the list. Before the update, it was putting to the bottom of the list.

I want it to be as it was before. Without the need to hold and pull it all the way down to the list.

And I want to add an important message for the devs. Please don't radically change things that people are used to. Or at least give an option to do it both ways.

Thank you.