[hyporthogon]

Not if the index is responsive to a Sydney/Bing request (which I imagine might be desirable for meeting reasonable 'web-enabled chatbot' ux requirements). You could test this approximately (but only approximately, I think) by running the test artursapek mentions in another comment. If the request is received by the remote server 'in real time' -- meaning, faster than an uninfluenced crawl would hit that (brand new) url (I don't know how to know that number) -- then Sidney/Bing is 'pushing' the indexing system to grab that url (which counts as Sydney/Being issuing a GET to a remote server, albeit with the indexing system intervening). If Sydney/Bing 'gives up' before the remote url receives the request then we at least don't have confirmation that Sydney/Bing can initiate a GET to whatever url 'inline'. But this still wouldn't be firm confirmation that Sydney/Bing is only able to access data that was previously indexed independently of any request issued by Sydney/Bing...just lack of confirmation of the contrary.

Edit: If you could guarantee that whatever index Sydney/Bing can access will never index a url (e.g. if you knew Bing respected noindex) then you could strengthen the test by inputting the same url to Sydney/Bing after the amount of time you'd expect the crawler to hit your new url. If Sidney/Bing never sees that url then it seems more likely that can't see anything the crawler hasn't already hit (and hence indexed and hence accessible w/o Sydney-initiated GET).

(MSFT probably thought of this, so I'm probably worried about nothing.)