You could also make the notes mandatory.

Or put the notes inside the slide, and then CSS is enough, the JavaScript doesn’t even need to know about notes:

  Notes:
  <label><input type=radio name=notes id=notes-hide checked> Hide</label>
  <label><input type=radio name=notes id=notes-inline> Inline</label>
  <label><input type=radio name=notes id=notes-only> Only</label>

  <section>
      Slide

      <aside>
          Notes
      </aside>
  </section>

  <style>
      section {
          position: relative;
      }

      aside {
          background: #feb;
          padding: 1em;

          body:has(#notes-only:checked) & {
              position: absolute;
              inset: 0;
          }

          body:has(#notes-inline:checked) & {
              margin-top: auto; /* concept: if the slide uses flex, notes can try sticking to the bottom */
          }

          body:has(#notes-hide:checked) & {
              display: none;
          }
      }
  </style>

I think one would want either slides or notes?

This seems perfectly heretical.

    <script>
    document.write(`
    <style>
    .${location.search.replace(/\W/g, '')}{
      display:none
    }
    </style>
    `)
    </script>