I received a clarification that custom-document-formats.json is only available since 15.9, so then redid the steps with a local modified document-formats.js :

• where I added according to the procedure, to increase the slide images.

"PRESENTATION": {"FilterName": "impress_html_Export", "FilterData": {"Width": 1024}}, 

• Saved the modified local file.
• Added it in \xwiki-platform-distribution-flavor-jetty-hsqldb-14.10.8\webapps\xwiki\WEB-INF\classes
• Started the wiki server (Jetty 14.10.8 as the client's version, having an Office server with LibreOffice set on it and connected).

And now I have an error in the startup logs: failed-to-parse-file.txt

2023-12-14 09:18:48,273 [main] WARN  o.x.o.i.s.DefaultOfficeServer  - Failed to parse /document-formats.js . The default document format registry will be used instead. 

showing that the file I added is not taken into account.