I find this improbable. Kessler syndrome could well make it impossible to keep a satellite in orbit at certain altitudes. However it is unlikely to create a “shell” that prevents space flight entirely. Kessler in LEO would be cleared fairly quickly (a generation or less) and at higher orbits the debris would be less dense due to the volume of orbital space it covered, which expands at the square of altitude. If we were really determined to get off the planet, I don’t think it would stop us.