This page is generally useful to Canvas template developers. Canvas generates a deprecation notice where the deprecated tag is used in a template:
{# base.html #}
{% deprecated 'The "base.html" template is deprecated, use "layout.html" instead. %}
{% extends 'layout.html' %}
Also you can deprecate a block in the following way:
{% block hey %}
{% deprecated 'The "hey" block is deprecated, use "greet" instead.' %}
{{ block('greet') }}
{% endblock %}
{% block greet %}
Hey you!
{% endblock %}
Note that by default, the deprecation notices are silenced and never displayed nor logged.