I tried to delete the page via the API Sandbox and didn’t succeed.

• action=delete with deletetalk= fails with the same error.
• action=delete without deletetalk= fails with an error “To delete a topic, use the ... menu on the Structured Discussions board or topic page. Do not visit action=delete for the topic directly.”
• action=flow + submodule=moderate-topic + mtmoderationState=delete fails with a permission error (apparently the Wikidata Staff group lacks the flow-delete permission).

You aren't supposed to be able to delete topic pages using core action=delete; you're supposed to use the flow APIs for that.

That said, since Flow is read-only on Wikidata nobody should be allowed to make any changes to it, including deleting topics.

Sure, the code shouldn't throw an error in the logs, but that's not worth fixing given the decaying state of the Flow codebase.

In T426566#11932004, @Pppery wrote:

That said, since Flow is read-only on Wikidata nobody should be allowed to make any changes to it, including deleting topics.

So we just have eternally frozen content on our wikis now? And if someone finds spam or PII in it that escaped notice during the sunset period, not even bureaucrats can do anything about it?

Sysadmins could do something about in a somewhat hacky manner, but otherwise yes.

Sidenote: the specific topic you are attempting to delete is already effectively deleted because the board it pertains to is deleted. As someone with no relevant perms on Wikidata, if I go to https://www.wikidata.org/wiki/Topic:Vzhs9x498f5cm1xc I see "Insufficient permissions to execute this action"

And while this is swatting a fly with a sledgehammer any admin can still delete (and undelete) entire Flow boards which causes all topics to enter the same effectively-deleted state.