What happens when an already synced directory is excluded?

Let’s say a sync partnership has folders A,B,C,D in it. It’s in sync with multiple devices.

Question 1: on a device with folder B synced, I exclude it using .stignore. On this device, what happens to folder B? Is it left in placed but ignored, or deleted?

Question 2: in another hypothetical scenario (not occuring after question 1), folder B is in sync on all devices. Now on every device, I exclude it. Does this mean folder B will be kept in place on all devices but ignored, or deleted from all devices?

Kept and ignored

Could you please clarify what would be the best way to rectify a mistakenly synced folder.

Let me use a practical example, let’s say a send/receive sync partnership is a home directory. I accidentally forget to exclude a folder that’s synced with other tools, such as a Dropbox folder, and now it’s synced globally. I realize my mistake, so I add it to a .stignore. But which option should I choose?

  1. add Dropbox to the .stignore on the originating device. Now, the global state is size X, and on one device, said originating device, is size X minus (Dropbox folder size). Then, I go to the other PC’s and delete the Dropbox folder.

  2. add Dropbox to the .stignore on all devices. Now the global state is equal on all devices, the size of which excludes the Dropbox folder. Then, on the PC’s that don’t need this folder, I delete the Dropbox folder.

I think both would accomplish the same thing. However, I am worried about option 1, there might be data loss. For example, after excluding Dropbox on the originating device, and deleting Dropbox on the other devices, syncthing might remember this deletion. In the future, if I removed Dropbox from the .stignore, syncthing might sync the deletion back to the originating device.

In typing this I think a send only/receive only combination might be more suited for this, what do you think?

I’d stop Dropbox, move the Dropbox folder out of the way. Syncthing detects the delete and everyone deletes the folder. Add it to ignore patterns. Move back.