After an installation or upgrade you can and should remove the following files:
I’m basing this on the post “Finalize the upgrade” from the Drupal documentation. I’m actually going to add a couple new files for the latest installations:
The basic reason for this is to attempt mask the Drupal version from the most common scanning methods.
Now, you can try and obfuscate your Drupal version but there is no way to hide the fact that you are using Drupal. This is mentioned in Drupal’s own documentation on the page, “Hide, obscure, or remove clues that a site runs on Drupal”, as well as a couple other sources on the internet:
So, it’s mentioned many times, in many places (I’m sure), but this is my own record of the files I can remove after a Drupal Installation (and note, I’m almost always referring to the most recent stable release).