The problem is that most (not all) of us don't use this name anymore, we use German name - Rostock. On Wikipedia you can see, that some people write it as Roztoka or even Rostok (??), but it's not standarized and official polish name is still Rostock (https://pl.wikipedia.org/wiki/Rostock). As you show, this is historical name and in OSM we map present times, not history. All names that are used these days can be see here, in the official document (and it's update from 2023): https://www.gov.pl/web/ksng/urzedowy-wykaz-polskich-nazw-geograficznych-swiata2

I would put "Roztoka" in old_name:pl, if it's important for you to have this name in OSM. It definitely shouldn't be in name:pl