If you’re open to third-party sync clients, Maestral supports this:

https://maestral.app/docs/mignore

I’ve been loving Maestral so far, it’s just the syncing, none of the other stuff. It has some downsides (it can’t upload symlinks but it can download them, and it doesn’t have LAN sync) but it’s super lightweight.

AFAIK Maestral doesn’t have block level diff and sync which is the main reason (for me) to use Dropbox: their superior sync engine

I have Maestral (and rclone) running on my raspbi but wouldn’t use it for things like sparsebundle syncing