I have several devices with companion app, but not all of them are personal mobile phones (some are just tablets with limited dashboards for everyone to use), so tracking the zone itself wasn’t an option for me. Instead I created an automation that only considers users from defined list. I decided to make it into a blueprint to share.
There’s also another blueprint for When First Person Arrives, if you need that.
If you use it and it works for you, please consider leaving a 
or a comment. Thank you 
blueprint:
name: 🚪 When Last Person Leaves
description: |
* Perform an action when last person leaves a zone.
* All the logic is incapsulated within the blueprint.
* You can add additional conditions if you need to.
domain: automation
author: oneplusuniverse
input:
zone_input:
name: Zone
description: Select a zone to monitor
default: "zone.home"
selector:
entity:
filter:
domain: zone
person_input:
name: Entities with location
description: Select entities to track
selector:
entity:
multiple: true
filter:
domain: person
additional_conditions:
name: Conditions
default: []
description: Any additional conditions
selector:
condition:
actions:
name: Actions
description: Add an action to run when last tracked person leaves selected zone.
selector:
action:
variables:
person_list_variable: !input person_input
zone_variable: !input zone_input
trigger:
platform: state
entity_id: !input zone_input
condition:
- and: !input additional_conditions
- alias: Internal logic
condition: not
conditions:
- condition: template
value_template: '{{ (person_list_variable + state_attr(zone_variable, "persons")) | count != dict.fromkeys(person_list_variable + state_attr(zone_variable, "persons")) | count }}'
action: !input actions
mode: single
