Triaging...

For v10:
Any block can be in the 'cell' position.

For v11:
Blocks can be set as BLOCK_POSITION_CELL in admin_setupwizard hooks. Only the main_poll block is set like it. That's not helpful, as you'd need at least two.
The fix is select other blocks should allow it too, such as main_content. Blocks where it really is designed to fit in a small space like that.

Some context:
Modern layouts and responsive designs don't tend to columnize things. We use responsive design to shrink things down based on viewport width. It would be possible to change our code to look at container instead of viewport (see https://caniuse.com/css-container-queries); but that would muddy our code to support a case that most people think is old-school crowded design.