Comment by Jtsummers

Permitted? Technically, it can't be stopped (well, it kind of can, see "Tolerated?").

Expected? No, it's counter to the intentions of the community.

Tolerated? Maybe. Here's a fun one (saw this in one of the past discussions dang linked): https://github.com/makew0rld/amfora/issues/199. That was over favicons, but the response would be similar if linked images were automatically fetched. Though the protocol has a "backoff" return code that could be used to throttle those things that would be less disruptive than Drew's approach of banning specific clients.