Does Syncthing have something like a "file source"?

What I would like to do is explain to Syncthing that my phone will create new files (photos that I take) and that it should receive those files, but when I delete them on my phone they shouldn’t be deleted anywhere else.

So, it’s not receive-only (which also receives deletes) but somehow more fine-grained. It would have to be receive-only-creates or something.

I guess it doesn’t exist and the workaround is to use file versioning somehow, right?

Yes, that is not an intended use case. If you feel adventurous, there is an advanced option to ignore deletes in a folder, but it’s more of an anti-feature and not well supported.

… and the folder will permanently indicate status “Out of sync”.