I’ve been using GMT for a long time and have gotten very used to the (now called) classical mode. Now that the modern mode has arrived, I’m wondering - should I leave the classical way of doing stuff behind and only focus on modern mode. Is modern mode the future? Will classical mode ever be deprecated?
I would say, give a try and after a while decide which one you prefer. Note that there new things in modern that you cannot do with classic (subplots, autolegend, insets, movies (I think)).
As for deprecating classic, no. We would not even know how to do that. Modern is just a clever layer on top of classic.
The good thing about the modes is that you really have to choose. You can mix and match classic and modern (even in the same script). So you can safely upgrade to 6.0 without having to go all in on modern mode. But I would definitely recommend trying to use it for new scripts you write. Your old scripts should still run fine.