I find it a little bizarre why they haven’t tested this before 4.3 was released.
I can say reading your remark that you are not so familiar with how WP likes to spring things upon the world without notice…
We have been testing both our own themes, and a huge group of plugins using the bleeding edge nightlies since WP 4.3 RC1 came out, right up to 2 nights ago. With the Rev Slider loaded, as we often contribute to them with bug fixes and suggestions.
Guess what? You only get one guess btw… No problems with the nightlies.
But come release time, and as usual, all those little things WP slipped in or never ran in the nightlies come to rear their ugly head.
And this is nothing. The change to the widget constructor contained in this release is going to affect thousands, I repeat thousands, of plugins and themes. Its only a notice error, but if you have debug on, your dashboard is going to go nuts. There is a track in the developer codex which indicates the WP team knows at least 4400 plugins just in the free codex which now have to update!
Care to guess how many themes and plugins here at TF have to update as well for the widget constructor change?
Don’t be so quick to judge ill of developers, because a hiccup occurs. Sometimes, especially when it is WordPress at the “core” of matters, everyone is a victim!