Check the comment from @NickD, regex:

    :base-extension "css\\|js\\|php\\|jpg\\|png\\"

is indeed wrong if we follow the tutorial exactly. I removed the last `\\`. Still, as I said, this was working before. The actual error was that a package named `cl` is not supported by Emacs anymore. For that, I had to see which packages are depending on it and I evaluated following on `*scratch*`:

    (require 'loadhist)
    (file-dependents (feature-file 'cl))

and saw that the package `htmlize`, which I use for org publishing was depending on it. The newer versions of this package are updated to work without the `cl` package. So, I had to reinstall `htmlize` basically and things started working again.