Are there any plans to add a --filter parameter to git gc as well,
that would be able to prune past history of objects and convert them
back into pack promises?

This operation will change the hash, and hence the history. I tried to draw attention to a specific trick:

https://public-inbox.org/git/25196441572039199@iva5-58d151f416d2.qloud-c.yandex.net/T/#t

but he was not supported.

-- 
zvezdochiot