It means that it is intended to be dropped completely from the next version. The purpose of deprecating something is so that people have time to go through all of the spots where they currently use the deprecated code and change it before the next version comes out.
The purpose of transitional doctypes is to allow people to transition from one version to the next. The HTML 4 transitional doctype provides a way for web pages using HTML 3.2 to start using the any new HTML 4 tags while gradually getting rid of all the HTML 3.2 tags that have been deprecated. The problem is that people are creating new web pages using the transitional doctype which was not what it was created for. New pages should be created using the HTML 4 strict doctype as there is no reason whatever for using tags that were flagged in 1997 as being obsolete and to be removed from the next version.
Unfortunately those creating HTML 5 have decided to riscind the 1997 decision to remove the unnecessary garbage tags.
As for target="_blank" - the reason it was flagged for removal from HTML back in 1997 is that it reduces your visitors options as to where the link should open. Without that attribute your visitor can choose whether they want the page to open in the same window, a new window, or a new tab simply by right clicking on the link and selecting one of the three options. Adding that attribute removes one of the three choices while still not giving the web page author any control over which of the other two options will be applied meaning that neither party has control over where the window opens any more.