Floor outlets (supply air) under sofa and desks?

Yes, and all the dirt falls into the floor outlets... great! And metal grates in the floor are the visual overkill for me. I'd rather have yogurt cups on the wall

all the dirt falls into the floor outlets

That is not the case (own experience).

Floor outlets for supply air are installed with us. However, we also heat through them.

No dirt, if children sometimes throw something in, just lift the grate and take it out.

Draft. Now in summer, when only fresh air comes in, I have to stand barefoot on it to notice anything. In winter at full load (heating function) there is a draft. That shouldn't be the case for you.

I find floor outlets less disturbing than wall or ceiling ones. Cleaning is possible without a ladder, although the exhaust air filters need to be cleaned more frequently and they are at the top.

One should perhaps consider that bed and sofa do not always come with slender legs... that suddenly limits new purchases... I would make [Wandauslässe]. I would also be bothered by the disturbed area on the floor, it somewhat resembles a manhole cover.

Floor outlets are located at and below the windows, where no furniture other than desks is placed. If you are not looking directly at the floor, you tend to overlook them.

Well, opinions are that different.

I also find floor outlets more comfortable in terms of cleaning effort.

They are always clean because you vacuum, etc.

Walls and ceilings, on the other hand, are usually only cleaned 1-2 times a year when the filter is replaced. This can sometimes lead to a gray edge forming there.