I am a hydrographer (sea floor mapper), not a historian, but I feel that I can give an in depth answer to this question that is worthy of this sub's standards.

I promise I'll get into several historical examples during my response, but first I need to address "accurately mapped". How should accurately mapped be defined? For something to be accurate, how far off can the lines on the map be from reality? A mile? 100 meters/yards? 10? 1? Do you need to be precise to the centimeter/inch? Does the height of an object matter, or should you only care about the horizontal?

Responses to these questions are generally a mix of two categories (1) "the best possible with the current technology" or (2) "the cheapest possible to fulfill project goals". For example we can look at NOAA Chart 12326 - Approaches to New York - only about a third of the chart is mapped with full bottom coverage within the past 30 years. In the other two thirds, the measurements are older and less dense (because they're done with older tech such as single beam sonar and even lead lines). But this is acceptable because the purpose of the chart is to help ships safely into and out of New York Harbor. The areas without full bottom coverage are either deep and off-shore with no concern of running aground or they are so shallow that they're non-navigable.

Even with satellite imagery, we're still working on having more effective maps to this day. In my specialty it's because the tech is getting better and allowing us to map more effectively (autonomous vessels!) and just as importantly, the ships are getting bigger, with deeper drafts, and going more places. But other mapping specialists are concerned with delineating wetlands or surveying buildings, etc. For all of these satellites are only part of the solution for getting the most precise information; you still need people on the ground.

Second point before I get to discoveries, the earth is constantly changing to this day. Storms shifted the sand on the outer banks of North Carolina to create Shelly Island, which was subsequently destroyed in a storm a year later. A volcanic eruption created the island Hunga Tonga-Hunga Ha'apai in 2014. Cliffs can collapse into the ocean and build out the shoreline. In Alaska, glaciers are melting, so the coastline is falling back, etc, etc.

Okay, now that that's said, let's get to the meat of the question. Have we found islands because of Satellite? Yes! In 1976, scientists observed a small island off the north coast of Labrador from imagery from the Landsat satellite. They named it Landsat Island in honor of it's "discoverer". Verification of the discovery was still done in person, and appears to have been as a perilous of an adventure as any other discovery:

After Landsat Island was detected on the Landsat 1 image, the task of verifying its existence was given to Dr. Frank Hall of the Hydrographic Service—a task that turned out to be more challenging than first thought.

As told by Scott Reid during a Canadian Parliamentary debate, Dr. Hall "was strapped into a harness and lowered from a helicopter down to the island. This was quite a frozen island and it was completely covered with ice. As he was lowered out of the helicopter a polar bear took a swat at him. The bear was on the highest point on the island and it was hard for him to see because it was white. Hall yanked at the cable and got himself hauled up. He said he very nearly became the first person to end his life on Landsat Island."

However, like most of the islands that have been created or discovered after the age of sail, this island was very small. Of course there are many geographical features other than islands in the world. The uniquely untouched rainforests on Mount Mabu in Mozambique come to mind as a recent satellite "discovery" - certainly it was known about by the people living in the area, but it was extremely inaccessible. It's observation in satellite imagery has allowed scientists to research it. Similarly, satellite imagery has been incredibly useful for archaeology with discoveries ranging from Mayan ruins, Norse hillforts, and politically inaccessible sites in the Middle East.

In short, satellite imagery is an incredibly useful tool for helping us understand the world, but the data we get from it does not equate to a full understanding. We still have a huge amount to discover in terms of updating maps to reflect increasing demands for information and our technological ability to deliver on those demands. Contrary to popular thought, with all of the changes in the world, the amount left to discover is only increasing.

Edit: correction to Labrador, not Newfoundland, thanks u/CubicZircon !