Allotments is different from a dacha, if I understand the concept of dachas correctly.

They are very small plots of land. I think dachas have more land... (??) And I think there is proper little cottage at the dacha, where you could stay the night, right?
That is not allowed at the allotment.

You could walk across an allotment in 10-15 seconds. It's maybe 10x10 metres at most.
People grow vegetables on them.

They were more popular in the past when people had more time and less money and really benefited from growing their own vegetables.

Allotments were let to people by the local authorities, if they were interested in growing their own vegetables. It was mainly used by working class people in larger cities. They spend the day at the allotment and then go home to their flat and sleep there.

The price is quite modest, but the allotments cannot be sold or sublet and if the tenant doesn't use it, they lose the right to rent it.

People could put a very small shed on the allotment to keep their gardening tools in. But it is not allowed to sleep there.

This is more or less a social democratic idea... It made a big change for workers, and later it became a popular hobby for workers living in the cities. It got popular at the same time as social democracy.. SD voters are the traditional users...

When my grandparents got engaged, both their families were against the engagement for various reasons. So they ran away and stayed in shed in an allotment that belonged to someone they knew.