Also, for Windau/Ventspils, the former is listed as the German name.

The page in question doesn't show anything when it comes to "Vilnius", "Wilna", etc when searched

Exonyms for larger cities such as Narva, Talinn, Riga, Vilnius, Kaliningrad etc are actually more common than endonyms. Most sources report Vilnius as Wilna, Riga as Riga, Kaliningrad as Konigsberg, etc. For villages/small towns I agree that those names are no longer in use, but for cities they are.
"Narwa" fully qualifies here. It's only one letter that fixes phonetics in German + this is a valid transliteration of the Russian name, and, according to Wikipedia, 87% of the city's inhabitants are Russian.
If "name:de" concerns you so much, then why not tag it under alt_name:de as in Klaipeda/Memel?

"Wikipedia was also discussed already multiple times. It just records some old exonyms but at the same prefers endonyms for places in question."
As for that, the german wikipedia has stated that the title always has to be the endonym and the actual german exonym to be highlighted, for example: Vilnius, deutsch Wilna. From what I can see here, "Wilna" seems to be the exonym.
"Sure, we already know you live everywhere where you need to claim to have local knowledge (Otzi also claimed to live in Poland, and in Klaipeda area, and in Kaliningrad as Иван2026 etc.)."
I don't actually have local knowledge of the area. I just check sources such as Wikipedia, Duden, etc. If you see my edits then they are mostly fixing stuff around my hometown of Bydgoszcz, because that's where I live. I also have GPS traces and StreetComplete editions as proof, see my alt account: @olas%20%E2%88%92%20alt%20konto%20do%20uwag

Firstly, I live in Poland, Otzi lives somewhere around Frankfurt.
Also Wikipedia, the most popular encyclopedia, has german names separated and in bold, so I wouldn't consider those names "random".

"new sockpuppet accounts"
My only accounts are the one I am using right now and my alt account that I use StreetComplete with. Who do you think I am, Otzi?
" I don't think it is my responsibility to sort this out"
If you want to sort this out then go ahead, do a global revert of multilingual names.
Old exonyms are for example "Kamienica Saska" for Chemitz, which I have already tagged under old_name:pl. But exonyms for larger cities such as Vilnius (Wilna in german), Riga, Tallinn, Daugavpils (Dunaburg) are still in vigorous use by travel agencies, TV channels, ordinary people, see https://www.reddit.com/r/WidacZabory/comments/1udr04i/comment/otm7x0i/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button https://ciekawostkihistoryczne.pl/2026/06/05/konigsberg-kaliningrad-miasto-na-krancu-swiata/ https://easteurotopo.org/indices/view/view.php?map=kdwrindex&section=Dunaburg etc. If that bothers you so much, why don't tag those names under name:de and endonyms under official_name:de?