Just to let you know, this now seems to work again.

So do I understand correctly that we're actually hitting a hard-coded limit in PHP which cannot easily be changed? I guess patching PHP to set the limit to 200 or 250 is not an option, right? Is there anything we can do about this at all?

I'm sorry if this is a stupid question, but: for all of my photographs uploaded to commons on 21 September 2019 or later (for example https://commons.wikimedia.org/wiki/File:Argenvors%C3%A4%C3%9F_von_S%C3%BCden_2.jpg) a lot less EXIF data is recognized than for my photographs uploaded on 16 September or before (for example https://commons.wikimedia.org/wiki/File:Wiesbadener_H%C3%BCtte_von_Nordwesten.jpg). Most notably, the GPS coordinates are not recognized any more. Could it be that it's a bug introduced with this change?

Ouch. I hope this can be changed soon, I think that many practical usages of Wikidata in the Wikimedia universe are currently not possible because of this issue, and I also think that even the current Wikidata usages consume magnitudes more resources (CPU time, disk access, electrical power, network bandwidth...) than what would be necessary.

No, it has to load and deserialize the whole entity (though it only returns a small part of it), since it’s all stored as one JSON blob.