i think someone on this site defined it best:
"poverty/disaster voyeurism"

IMO

Space that is adapted over time primarily by its inhabitants, rather then buildings and cities that are planned and constructed in one giant swoop by a single, removed entity.

Informal architecture does occur in areas stricken by poverty or natural disaster, but those elements are not at the heart of what makes it unique.

Quinta Monroy / Elemental Chile. Social Housing.

This project is a good example of how architecture can be adapted by the users over time. The great thing about this one was that the designers planned for this, and the architecture empowers the residents to have a hand in creating their own environments. I considered the stuff built between the main structures over time to be the the"informal" part of this project.

informal architecture?   Miscarriage of architecture, by means of low productivity in wealth and progress...

As in skinned furs of teepees and canavas.  Modernism of formal architecture must meant the deeper stone blocks which create such churches.

It may mean pre-existing sketches and drawings to the future sites of the chosen.