Dfyjkt

This question is inspired by this meta answer.

In late 2015, a number of Schengen countries introduced border controls in response to the migration crisis.

This answer succinctly explains what these border controls do and don't mean. It's clear that there can be some interruption to travellers, but also that border controls doesn't mean a closed border.

This question is:

Which Schengen border crossings still remain controlled due to the migration crisis?

Is there a way to keep up to date on this as it changes - particularly during travels - other than trying to keep on top of any developments in the news?

It's nigh impossible to know this without extensive recent experience with a specific border. Even news reports can be misleading and there is no easy way to keep up with the situation.

There is a procedure for a member state to officially notify the EU Commission that it is reintroducing border checks (cf. title III, chapter II of the Schengen Borders code), which is probably the basis for many maps and articles you will find on the net. A list of states that used the procedure is available to the public on the official EU website. But whether states did it or not does not really reflect actual practice on the ground.

For example, I have heard many direct and indirect reports from people who crossed the border between France and Italy and witnessed border checks there but, to my knowledge, France never notified the EU about it and the situation has been going on for much longer than envisioned by the regulation.

On the other hand, France did report its intent to reintroduce border checks for the Paris Climate Conference last December but even after the Paris attack that happened at the same time and the ongoing state of emergency, the checks are far from systematic.

And when states do reintroduce border checks, legally or illegally, they typically focus on specific locations, targeting sensitive borders, larger crossings or public transportation networks rather than actually staffing each and every border crossing point (many of which don't even have a building anymore). I believe many Schengen countries have reduced the size or refocused the tasks of their border police force and simply do not have the capacity to perform full border checks without seriously straining their regular police force.

The most updated image I could find is this one: (EDIT: though it is not the most updated information you can find online)

Meanings of colours:
Green = no check,
Orange = checks at their borders,
Red = borders temporarly closed

Since, this image was made (on November 12th), the terror attack in Paris happened so France should be orange : random checks at their borders. (government said it is closed but I actually go through the border with germany often and it is just a joke...)

EDIT: this map shows the exact closed / checked borders on the migrants route to Germany. It is not providing any other information about other routes (to nothern countries per example). Map has been made in March 2016.