Post types losing feature support must have it ignored for generation

[sybrew]

When a post type loses supporting a feature, existing data still stored in the database should be ignored. These data cannot be altered within default WordPress interfaces.

So far, I accidentally came across excerpt ($post->post_excerpt).

This might also affect the following:

1. title (->post_title)
2. editor (->post_content)
3. author (social sharing metadata)

There might be more, such as attachments. Further investigation is necessary.

[sybrew]

Little in WordPress checks for this -- it is also regarded as a defect therein: https://core.trac.wordpress.org/ticket/45516.

Fixing this requires only a lightweight check, so I will push through to have this bug resolved. WordPress can resolve this on its own time, probably after Gutenberg is out of alpha.