Problem/Motivation
ajax_page_state is a list of the 'minimal representative set' of libraries on page, which can still be a long list. We should compress it, this will allow it to be passed into GET requests per #956186: Allow AJAX to use GET requests.
The mechanism for this has already been added in #3303067: Compress aggregate URL query strings.
However, we need to figure out whether bc is necessary - some contrib modules might inspect the array. One possible way would be to decompress the query string and replace it in the incoming request, essential upcast it, but... not sure what that looks like yet.