Short answer
HubSpot's KB: workflow sequence enrollment is limited to Enterprise; Sales and Service Hub Professional can create workflows from the sequence tool's Automation tab. If a user exceeds their daily send limit, the contact skips the action. Contacts can be in one sequence at a time.
1. The actions
Searching the action picker in my demo portal.

2. Deleted sequences
If the sequence is deleted, the workflow stops enrolling contacts in it.