In documentation for python 3.3, the deprecation warning is rendered in a red box which is more eye-catching.

eg https://docs.python.org/3.3/library/imp.html?highlight=imp#module-imp

But since python 3.4 onwards, seems like the red box disappears and the deprecation warning message no longer stands out.

for example
https://docs.python.org/3.4/library/imp.html?highlight=imp#module-imp

I kinda prefer like the older style (with the red box).

default.css was changed?
https://docs.python.org/3.3/_static/default.css
https://docs.python.org/3.4/_static/default.css

It was rendered in a red box, because we were using an old Sphinx CSS file (Doc/tools/static/basic.css) [1] We are using Sphinx defaults now and Python specific tweaks are located at Doc/tools/pydoctheme/static/pydoctheme.css.

We usually prefer to use less disruptive visual elements (including using less gray note directives) in Python documentation. The exception to this is security warnings. See https://docs.python.org/3.5/library/xml.html and https://docs.python.org/3.5/library/ssl.html for example.

I recommend to close this as "wont fix".

[1] Removed in https://github.com/python/cpython/commit/7c366a72ac56814edaf3fb2718b08441aff31d3a

Agreed. The removal of the red was a conscious choice.  As I remember it, that choice got reflected into the Sphinx defaults, rather than the other way around :)