Funny, I'm still using the same MySQL that I've been using for the last 5 or 6 years.
Oh, granted the MySQL 5.5 added some features such as partitioned tables. But you don't have to use those features if you don't want to.
But of course, you don't really mean that anybody deprecated MySQL. You apparently are a PHP coder with a narrow view of the world, yes?
So what you really mean is why did the PHP people create a new *LIBRARY* for accessing MySQL? It couldn't possibly be because the old one was horribly inadequate, could it? After all, Java (JSP) and C# and VB.NET and even VBScript users haven't lived with a lot of the limitations in the old PHP library for many years now. (To name just one example, all of those that I mentioned can send multiple queries to MySQL in a single command. The old PHP mysql library didn't allow that for some silly reason.)
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.