Can I follow or watch a thread without being assigned to it?
No. Plain has no follow or watch feature for threads. The only way to get notifications about a thread is to be assigned to it, either as the lead assignee or as a co-assignee.
Personal notifications cover only threads you are assigned to. When you are assigned or added as a co-assignee, you get notifications for new messages, mentions and status changes, based on your notification preferences. To change these, go to Settings, then Notifications.
Add yourself as a co-assignee
If you want to stay informed about a thread without owning it, add yourself as a co-assignee. You get notifications without being solely responsible for the thread.
Co-assignment is visible on the thread, so other team members can see you are involved. There is no way to observe a thread without a visible footprint.