Acrylics in the drain? In an imaginary scenario, I paint with acrylics, let's say on canvas board or similar.
I don't use any fancy techniques, like impasto. Just the &quot;normal&quot; amount of paint and commonly used acrylic mediums.
I would like to know, whether I should be concerned about what these materials do when they get into the drain pipes (in the kitchen sink, or in the bathroom), when I wash out my brushes, and maybe some jars.

I know that as soon as acrylic painting materials get wet and then get a chance to dry, they end up forming a solid substance.
As far as I know / can imagine, drain pipes are not always soaked 100% in water, rather they have patches that get non-submerged from time to time.

Is it possible that acrylics materials could form dried-on layers within the drain pipes? That they could build up with time, decreasing the cross-section available for water flow?
Is this a valid concern? If yes, what could I do to mitigate it?
Yes, that concern is very valid, and it has always been a problem in any school art room I've ever been to (even though we used only water paints in school).
You should never wash big amounts of paint down the drain. The safer method is to simply let it dry and dispose of the remains in the household waste. Bigger amounts should be left to dry in a disposable container or plastic bag. Smaller amounts can be transferred to some old newspaper, paper towels, or similar disposable surface.
Brushes and mixing palettes should be wiped clean with a paper towel and then rinsed in fresh water. After you finished rinsing, continue letting the water flow for another minute to wash all the residue down your pipes. This is also useful for rinsing the pipes of old soap or fat residues (for example, after washing very oily dishes).
One solution I personally use: My mixing palette is a common glazed saucer. I let the paint that I no longer need dry on the saucer and then soak the whole saucer in water the next day. The acrylic paint won't dissolve in the water, but it will release from the glazed surface, and I can easily peel the paint remains from the saucer and dispose of them in the waste.