@Andrew-WMDE if you introduce the fix with this ticket, can you close the related ticket as well? gracias

I was just about to deploy this change but I realised that before I did there seemed to be some questions I couldn't instantly see the answer to.

From the task description:

Figure out if we can hide it or if we need a process for removal

Did you conclude that it wasn't possible to hide and therefore we needed to remove the whole extension?

I guess we should think about what those steps might be in that removal process. Some stuff we could consider:

• Do any of our users use this extension?
• What was the reason it was added in the first place?
• Other than fixing this issue does this have any other impact?

Also:

Investigate where this is coming from / what is broken

Did you come to any conclusions about this? Was this extension just never supposed to work with Wikibase?

So it was decided that we were indeed fine to just remove this whole extension. This happily removes the error popups seen in T327235 and as a side effect also removed the normal popups you might see in Wiki links giving a preview of the article e.g

Did you come to any conclusions about this? Was this extension just never supposed to work with Wikibase?

I believe the conclusion to this is: yes, it was never designed to work on Wikibase. It was built for and mostly deployed on pure wikitext wikis (makes sense since it's developed by the readers web team)

Specifically T111231 exists for the purpose of adding popups summaries for Wikibase and in an ideal world would have been implemented, but without it items and properties will at least show a label and description which is better than now. (Lexemes and EntitySchema lack this.)

I think reference tooltips will degrade to links to the references section but I could be wrong.

The specific task for the summary endpoint is T199096.