Using Inkscape 1.3, I cannot find where to apply a "Simplify" path effect. Nothing comes down on "Add Live Path Effect" menu, nor is there any Apply button. See attached screenshot.
In Inkscape path effects can only be applied if you select a vector object first. If you don't have one selected, then it won't show any path effects or even let you hit the menu button to get to the dropdown. Also it won't work if you have selected a raster image (because it's not vector). It won't work for a text object either, unless you first convert it to vector outlines using Path > Object to Path.
There is a trick/hack for text objects though, without having to convert to outlines. If you clone the text, you can apply a path effect to the clone.
The Simplify Path effect is shown here