It is also possible to add an option of maximum disk size per folder. Like the option minimum free disk space.
Many things are possible. What’s the use case?
To give other family members (children) a reserved space to use my computer as a backup. And to prevent them from filling up my entire computer. So that I keep a little control over my disk space
Workaround could be some additional partition or container for those folders, but having an option in syncthing would make it easier.