Entries retain field data that has been removed from entry type layout
Technically, all entries have all fields, regardless of their layout. Field layouts are mostly just a UI thing.


  1) Is there a way to tell if an field is actually actively assigned to the entry layout?


It’s a little cumbersome and would require the MN Break and Continue plugin, but yes:

{% for flf in entry.getFieldLayout().getFields() %}
    {% set field = flf.getField() %}

    {% if field.handle == 'text' %}
        {{ entry.text }}
        {% break %}
    {% elseif field.handle == 'multiContent' %}
        {{ entry.multiContent.text }}
        {% break %}
    {% endif %}
{% endfor %}



  2) I understand why the field data would be retained in the database even though the field was removed from the layout, but under what circumstances would populating 'deleted' fields for entries be desired behavior? It surely adds overhead, it is counter-intuitive (imo) and has the potential to cause all sort of issues.


Actually it’s faster for Craft to not worry about the field layout, which is why it’s not taken into account right now. Less data to fetch, and logic to run. If you check for entry.text, Craft just assumes you know what you’re doing.

Maybe a better approach would be to just check the entry type’s ID/name:

{% if entry.typeId in [1,2,3] %}
    {{ entry.text }}
{% elseif entry.typeId in [4,5,6] %}
    {{ entry.multiContent.text }}
{% endif %}


(or)

{% if entry.type.handle in ['one', 'two', 'three'] %}
    {{ entry.text }}
{% elseif entry.type.handle in ['four', 'five', 'six'] %}
    {{ entry.multiContent.text }}
{% endif %}