Sensor since last changed using relative time?

Configuration
16

Try this:

message: |
  Some devices haven't been seen lately:
  {% for state in states.sensor -%}
    {%- if state.attributes.linkquality %}
      {%- if  "linkquality" in state.name and (as_timestamp(now()) - as_timestamp(state.last_changed) > (12 * 60 * 60) ) -%}
           .       {{ relative_time(strptime(state.attributes.last_seen, "%Y-%m-%dT%H:%M:%S%z")) }} ago for {{ state.name | lower }}     .
      {%- endif -%}
    {%- endif -%}
  {%- endfor %}

This works because relative_time is expecting a datetime object, and the attributes are stored as strings. You apparently need to convert to a datetime, and the only way I see available to do that in the template is via strptime.

EDIT: Updated to use last_seen in both places and account for lqi=0.

message: |
  Some devices haven't been seen lately:
  {% for state in states.sensor -%}
    {%- if not state_attr(state.entity_id, 'linkquality') == None and state.attributes.last_seen %}
      {%- if  "linkquality" in state.name and (as_timestamp(now()) - as_timestamp(strptime(state.attributes.last_seen, "%Y-%m-%dT%H:%M:%S%z")) > (12 * 60 * 60) ) %}
         {{ relative_time(strptime(state.attributes.last_seen, "%Y-%m-%dT%H:%M:%S%z")) }} ago for {{ state.name | lower }},
      {%- endif -%}
    {%- endif -%}
  {%- endfor %}