The Kessler syndrome is when the low Earth orbit is so full of fast-moving space junk that the orbit is unusable, and space travel has become too dangerous to be feasible.
This was just a theory when it was presented in 1978.
In 2009, he warned that the debris environment had already become unstable.
Since then, the number of objects in LEO has increase a lot with constellations of small communication satellites such as StarLink.
Satellites and the space station regularly have to use maneuvering thrusters to avoid space debris.
Last year, scientists published a warning that a solar storm could knock out satellites' ability to evade space debris: and as little as three days of downtime could allow cascading space debris collisions to lead to the Kessler Syndrome.
I think we instead need a global moratorium against these kind of satellite constellations, until such a time that the space debris has cleared.
Launching them would be highly irresponsible.