Addressing your literal question: No.

Legend is extremely versatile so most features aren’t set in stone as to what they “do”. It’s very much up to you and how you use them. There is, for example, no explicit “bookmark” feature as you have noticed. This makes for a very powerful app – but a nasty learning curve. It’s worth it though, if you stick with it.

A Board is not a “bookmark”; they are very different concepts.
Boards collect together sets of Panes, which makes them function a bit like “desktops”. That’s how I think of them – but everybody finds their own metaphor to relate to.

The Panes within a Board offer customizable views of a document, including not just the position you are zoomed to, but how the contents are arranged, sorted, grouped, and filtered. If you switch to a different Pane or even to another Board, the Pane you left behind will remain the same when you return to it. Because of that, a Pane can be used somewhat like a “bookmark”.

Some people like to create and delete Panes dynamically as they work. If you are like that, then Jay’s suggestion above is helpful: When you’re working in a Pane and decide you want a “bookmark” of that view, clone the Pane to a new one. Perhaps edit the clone’s name so you can easily tell it’s a bookmark (you can begin the name with a 🔖emoji to help with that). Then you can return to the original Pane and freely navigate around within it, change filters, or even close it – whatever you like. The cloned pane – now a “bookmark” will not change when you do this.

Having said that, the “bookmark Pane” does NOT lock down the content of your document; just the view. Every Pane pointed to Document A is simply a view of the same single underlying document. If you make a change to the items in Document A, every other Pane which is pointed to Document A (including ‘bookmarks’) will reflect those changes. It’s like looking inside a building through different windows. It doesn’t matter which window you look through; you’ll see the same contents inside (just from a different perspective).

So one thing to watch out for is if you have a “bookmark” Pane zoomed into an item and then delete that item from some other Pane. When you return to that “bookmark” you will probably not see what you expected.

Hope this helps!