I'd like to draw attention to a page in the wiki  that summarizes all of the planned deprecated settings, features, syntax, etc. that are scheduled at present to be removed in IdP V4 in the future.
This is a summary derived from the warnings added to the eventual IdP V3.4 release, which will call attention to their use and allow people to adjust any legacy behavior far in advance of the V4 release, which will make the eventual upgrade fairly automatic.
It is probably not 100% perfectly accurate at this stage, but it should be very close, and will be maintained going forward, eventually making up part of the V3.4 and V4 release notes directly.
Content is somewhat subject to change until V3.4 ships, but unlikely to change much if at all after that.