As per title: I have a list with "favorite" markers for its items, I want to add a keyboard shortcut to toggle the favorite status of an item.
is there any typical keyboard assignment in commonly used, popular applications for that?
The shortcut to add a favorite in browsers is command+d (mac) or ctrl+d (windows). For applications in general, I suggest you have a look at https://defkey.com/. If you search for "ctrl+d" (https://defkey.com/what-means/ctrl-d), for example, it shows a variety of different meanings for different applications.
It would depend on the language you have already used. Im not sure if you have access to see Atalassin but their keyboard shortcuts appear almost random yet product managers seem to effortlessly remember them all. https://confluence.atlassian.com/agile066/jira-agile-user-s-guide/using-keyboard-shortcuts
Dribbble uses 'l' because they use 'like' as the Call to action.
In summary so long as they are sensible i.e. Favorite 'f' and you are clear that this feature exists, then there shouldn't be any confusion.