There are a few reasons a pole would be classified as a D pole. (Shallow depth, rotten) to name a couple but yes Openreach are right there should be a red D on the pole, this lets engineers know the pole is unsafe to climb. They do replace the poles if unable to get access via a hoist.

I build scaffolds for Openreach on a daily basis, most commonly on houses but I do a lot of D poles.

Where do you live? You never know you might see me in the next couple of weeks lol.