Stenosis or narrowing of the spinal canal can be treated without surgery. Medications and exercise are the first treatment methods used. This is called conservative therapy.
Surgery isn't conservative and is called invasive because it means cutting into the body. You're right about the trend toward less invasive operations. Smaller incisions can be used with today's improved technology.
The surgeon can use tools inside the body with tiny TV cameras on the ends. This gives a view inside the body without actually cutting open the spine. At the same time, surgeons know more about what works and what doesn't.
Smaller cuts and removing less tissue are just as effective as the more invasive surgery years ago. Instead of removing all of the bone or joint around the spinal canal, only part of the tissue is taken out. This is called a partial lamiarthrectomy. It replaces the full laminectomy and complete arthrectomy used a decade ago.