How to fill these holes

Absolutely do not use an expanding gap filler, it will make matters worse. If you have verified that each step is level so that it doesn't need to be re-secured to the tread than the gaps can be filled. Also verify that the steps are not loose or shifting.

The easiest and most used product to fill each gap would be a latex caulking. A cartridge (or tube) is placed in a caulking gun and applied in a continuous bead along the gap. Once the gap is covered press it into the gap with a putty knife. This will also flatten the caulking so that it conforms with the flat wall. A very damp rag will wipe up any wayward caulk that didn't get pushed into the gap.

It will take about 24 hours for the caulk to fully cure. It than can be painted if desired, but there are plenty of colors to choose from which may not make painting needed.

As been suggested by a prominent authority well versed in caulking and fully supported by myself, you must be sure to use a Latex-type caulk. Not to be confused with a 100% silicone or a urethane caulking. Latex is easier and more forgiving than the other types.

You will need to either replace the wall or build another new wall behind the original, thus rendering the old wall obsolete. Also there is a lot more to it than that, but this is the simplest way to answer it.