I believe I found something. The code sample from the broken links in js2shapefile is available in this GitHub:GitHub - borisdev/js2shapefile: Automatically exported from code.google.com/p/js2shapefile.
I forked the repo, downloaded, and this sample actually works: js2shapefile/Test_EsriDrawToShapefile.html at master · borisdev/js2shapefile · GitHub . Uses Draw Toolbar to create lines, points, and polygons and can save all at once into their shapefiles. Pretty cool!
After downloading the shapefile(s), you just need to assign them the Web Mercator Auxiliary Sphere projection. Once you have that .prj file, you can copy and rename to match others. Tool just creates .dbf, .shp, .shx files.
Code is 4 years old and uses ArcGIS JS 2.6, but is a simple app and a good starting point to upgrade to 3.16 and rework/modify the GUI. Would be good to be able to name shapefiles you save. Also needs to "Clear Graphics" button, only way to draw and export additional shapefiles is to refresh page.