I don't think any have been removed since the bookmark section was introduced. Same applies to Community, Reviews and Designs by Cuties. In the Project section the one who posted the project can remove (or edit) it, other than that it too stays.
See that list of pages at bottom of page on bookmarks. Page 143 is from March 08, so at least that long.
I like some of those really old ones so I was wondering how how long I had to figure out which ones I needed to personally bookmark or if they would just be there forever. Thanks!*4u
the ones i just looked at pages and pages down have been there along time so i dont think so