E9 (D2). Where did my presentation page went to after the update?

If after updating from a pre-1.9 version you can’t see your presentation page, go to the WordPress admin » Settings » Reading and switch the Front page displays to its default value which is Your latest posts.

This is no longer the case since 1.9.7 as the presentation page is now visible when enabled no matter the WordPress setting.
If you need some other page to show your blog posts just edit that page and choose Blog Template as page template.

Make sure your “Frontpage displays” is set to “Latest posts” under Settings > Reading.