See T58371 (and consequently T191722) for why checking the summary doesn't work. I still have to check if the EditFilterMergedContent hook provides enough context to determine whether the edit is creating a new section.

@Daimona I recently submitted T226563. If that task was implemented, would it provide a possible solution?

No, tags are applied after filters are executed.

I forgot to check the code, I'll try to do that tomorrow.

So I checked EditPage. The section being edited is saved in a class property, EditPage::section. Unfortunately, it's not passed as argument to the hook, and thus it's impossible to read it from AbuseFilter. The two solutions I can think of:

1. Add it to the hook params. However, this may not be optimal (lots of params...)
2. Read it from the request, like EditPage does: $this->section = $request->getVal( 'wpSection', $request->getVal( 'section' ) );. However, this could be unreliable as it would miss any value manipolation done by EditPage.