info from Github staff:
To make it easier to vendor third-party dependencies via package managers like Bundler or NPM (or Yarn), Jekyll now ignores the vendor and node_modules directories by default, speeding up build times and avoiding potential errors. If you need those directories included in your site, simply add them to your configuration's include directive.
just keep in mind, avoid directories with names: vendors or node_modules when deploy anything to gh-pages =)
other info from Github staff:
Since you're not using Jekyll, it will be easier for you to add an empty .nojekyll file to the root of your repository. This will tell Jekyll not to run, and then your vendored files will be included in the build.