If I write a post on this page (https://itch.io/dashboard/posts) without linking it to any project or devlog, will it be sent to all my followers' emails like a notification (as it happens with a devlog), to the owners of my project keys, or will it only appear in the feed of those who follow me on itch.io?

Posts that are not associated with a project will appear on the feed of those accounts that follow you. I believe feed posts are also eligible for email notification as well.