Of course there are workarounds. Bookmarking isn't great because that still takes you to the top level and you still have to navigate around to find what you want or use the built in search -- which is OK, but not ideal.
The biggest problem with workarounds is that they only work for me. Google (or search generically) could be argued to be the primary user interface for the internet. Returning version 1.0, 1.1, and 2.0 results as the top hits is IMO a bug of our website. If I have this problem, so does everyone else who tries to Google FLTK.
I have a bit of a hoarder mentality too -- particularly when it comes to data. I appreciate the desire to support everything going all the way back -- especially when it seems like it 'doesn't cost anything'. In this case, the cost of keeping the deprecated html documentation indexed on the website is that straightforward search is broken.
Sometimes it is best to clean up a bit. At this point in time, we don't actually want anyone using 1.0, 1.1, or 2.0. If you have a legacy program that can't move forward, then you can probably access the documentation from the source tarball available from the old releases download page. I think it would be entirely reasonable to delete the deprecated docs from the website -- marking them to not be indexed is a much smaller step that lets us hoard our data while fixing search.
Rob