Is it possible to take incremental snapshots without removing old data from previous snaps?

I would like to keep a remote repo of my most important files, which I cannot re-create by any other means.

As such, I would like to snapshot a directory, maybe /important, which changes. For example, write some files to that directory, take a snapshot, delete the files, and add new content, take new snapshot.
I would like the current snapshot to include both sets of files somehow, rather than snapshot 1 having the original stuff, and snapshot 2 missing those files.

Is this possible? Through some kind of clever scripting or union mounting or some such solution?

Kopia is a backup app - not an archive. Since retensions always apply to the complete repo, you also simply can’t use those to push deletion to infinity. You could to use a separate repo for that and then only snapshot what you want to keep “forever”, by pushing the retension way out. Due to the standard way, storage technology evolves, I’d say, that “forever” means approx. 5 years, until you will have to move your repo onto a different storage.