If you haven't read the Wikipedia article on deprecation, you might not know it's from the Latin word deprecare, meaning "to ward off (a disaster) by prayer." That pretty well describes the quandary for any software product that has a big established user base. Is it better to keep junk features, in case removing them will cause disasters for the old users? Or is it better to remove junk features, to avoid ridicule and to make the manual slimmer for the new users?
Historically MySQL has deprecated with caution, and usually MySQL has followed these steps:
(1) In version X, for any SQL statement that appears to use the feature, return a warning that the feature "has been deprecated and may be removed in a future release".
(2) Also in version X, put a notification in the manual. Notifications have a regular pattern so they're easy to find by looking in the "What's new" section, such as http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-4.html, and http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-4.html.
Alternatively, one can look for the "nutshell" pages, such as https://dev.mysql.com/doc/refman/5.6/en/mysql-nutshell.html and https://dev.mysql.com/doc/refman/5.5/en/mysql-nutshell.html. Or, in googlespeak, site:dev.mysql.com/doc "what's new" "deprecated". Notice that I am pointing only to the MySQL manual, because MariaDB does not appear to have an equivalent (the MariaDB "deprecation policy" page only talks about supported platforms).
(3) In version X+1 or X+2, actually remove the feature and start throwing syntax errors.
My all-time favourite deprecation was of the TIMESTAMP(M) data type before version 4.1. The M stood for "display width", for example, with TIMESTAMP(6) you'd only see the first few digits. We realized that was in conflict with the ANSI/ISO standard, so it became deprecated in 4.x, became a syntax error in 5.x, and then -- years after the original warning -- the syntax became legal again with a different meaning, so now the 6 in TIMESTAMP(6) means "fractional seconds". Because there was a period when the syntax was simply illegal, it was practically impossible to move an old application to the new format without noticing anything.
These are my nominations for favourite deprecations in 5.6 and 5.7.
SHOW AUTHORS was an "Easter egg". Naturally I reported it as such at the time because Easter eggs are more than bugs, they are vandalism. Eventually the feature got documented, but the smell of rotten eggs was in my nose for years afterward, so I'm relieved it's gone.
A bit of a caveat: I was totally against adding this feature, and some of my bias might show. But I’m not trying to argue about it. I do believe that once we add a feature we should explain it, and all I’m trying to do is explain here.
... which had nothing to do with the quality of the user contribution. People who know me as the original crusader for PERFORMANCE_SCHEMA might guess that I saw this feature as out of step.
INSERT DELAYED wasn't junk per se. The problem with little non-standard performance-enhancing niblets is that they're always non-orthogonal. People sense that. So the bug report / feature request for -- why not? -- "UPDATE DELAYED" got about 15 votes. Adding the EVENTS feature didn't quell the protesters, because any rational person could see that, if INSERT DELAYED was okay syntax, UPDATED DELAYED should also be. (This makes two posts in a row where I'm railing about lack of orthogonality -- I brought it up in my blog post last week about MariaDB's DELETE ... RETURNING feature. By the way, I updated that post to show what decent standard-fearing folk would do instead of copying PostgreSQL.) So, to stop the protests that there's a precedent, get rid of the precedent. Well done.
What these deprecations are telling me is that somebody at MySQL at last has the authority and determination to clean the garage. The product is not just getting new good features, it's losing old bad ones.