I feel that in my area the driving culture has become so toxic that there’s a better than average chance that indicating a lane change (which I always do) will lead to the vehicle in the lane you’re attempting to change into accelerating to prevent you from ‘getting in front of them.’
It’s so frustrating (and dangerous!). It seems that a lot of folks feel entitled to the road, or the patch of road in front of their car fro as long as the eye can see, and are willing to behave irrationally regarding it.

I feel that telegraphing that your vehicle is slowing down (for any purpose) will lead to overconfidence or even willful misunderstanding by other drivers. A careful slow-down will turn to panic as they try to take advantage of the situation. I also think that drivers will focus on the vehicles too much, and will not focus on things like pedestrians or perhaps why your car is slowing down, and wind up contributing to the problem.