The individual movements of the 8th Symphony don’t have a key assigned (as an attribute in the XML), but those for the 7th (and 9th) do. Both symphonies have a key assigned to the entire work.
What is the preferred style? (I happen to believe it should be assigned at the entire work level only; the key usually only refers to how the symphony opens and doesn’t stay the same across, or even within, movements).
I would only assign the symphony’s key to the main work. If the (main) key for a movement is known, which as you said doesn’t need to be the same as for the full work, I’m fine with that being added to the appropriate movement.
There was a mistake with 2nd movement of 7th symphony. I had added same key for all movements (D minor) but 2nd movement should be in F major. It’s fixed now. I also added keys for movements of 8th symphony.
I wonder if there could be a tool to batch-add keys (or already exists). I add lots of works since I try to keep Telemann works up to date and it seems that every recording needs a new work so I usually add keys only for the works, not the movements, except if the movement has a specific key (as it happens quite regularly with Telemann). Maybe there could be an imslp importer that could parse keys and maybe catalognumbers, composition dates, dedications, movements etc?
You might want to take a look at this script to set at the same time work types/languages/keys on a composer Work page. But please be careful and check your edits regularly because there are a lot of side-effects I might not have foreseen
As far as I know the IMSLP pages do not provide any clear data structure to find the relevant fields or parse them (dates can be incomplete, I don’t know if keys are always set in english, etc.)
So I’m not interested in working on this in the near future
If you are interested in adding keys for parts of classical works, you might find some info from acousticbrainz analyses of recordings (that you can display with this script on recordings overview pages)