What is this?

Stripping kitchen ceiling to repair cracks and re-skim before paint. What is this substance in certain areas ? It’s very smooth and very hard/brittle. It’s an old police house.

Found a piece of newspaper from 1968 in one poorly filled hole to guess that’s when the last bit of work happened on this ceiling.

Do I need to do anything special or remove it or can I skim over it?

Plan was to fill any holes/cracks then scrim/jointing tape and then skim over the whole ceiling before painting.

Advice appreciated, thanks :)