On Acrobat X I used PDFScript_FlattenPages.js to flatten stamps I would apply to a document. My question, where can I find a .js to do the same on Pro DC and where should I add it to show up on ProDc?
In Acrobat Pro you can use the Preflight Tool.
You can use the same script, just install it in the corresponding folder for Acrobat DC (the script does come with installation instructions, if you need the full package, you can download it again from here: Free Acrobat Automation Tools). The flattening tool will be part of the "Add-on Tools" (Tools>Add-on Tools):