“Include Anti Feature Apps” in Settings > App compatibility:

There you have to add the anti-features you may accept included in Apps. For OM you have to enable these:

(But not 100% sure about this. Maybe the first circled is also enough?)

You have to enable “Unwanted Features” in F-Droid settings before it shows up.

Link to the app: https://f-droid.org/packages/app.organicmaps/

Please note that Germany has (compared to other EU members) quite strict and company-friendly protesting laws.

Such protests may be even considered as political protest (Politischer Streik) which makes them not illegal per se but could be illegal. https://www.bpb.de/themen/medien-journalismus/netzdebatte/219308/ein-bisschen-verboten-politischer-streik/