I like any of the following

>
is deprecated as of Symfony 2.5

>
was deprecated in Symfony 2.5

The second sounds a bit more natural, only because of the "as of" in the first sounds "ok" to me, but I don't know if that's so easy to understand for non-native speakers (but I have no idea!). I guess this is also consistent with the "was introduced" we're using for new features. It makes it sound like the deprecation of addition of features happened at a point in time, which makes sense to me.

Another fun editorial conversation :) - I think they're all "correct", so now it's just a matter of taste.