I think you've mixed up what the react-router-dom v5 exact prop represents, or rather, that you've conflated the behavior between the exact and strict props.

v5

• exact: When true, the active class/style will only be applied if the location is matched exactly.
• strict: When true, the trailing slash on a location’s pathname will be taken into consideration when determining if the location matches the current URL. See the <Route strict> documentation for more information.

Note the emphasis regarding exactly matching is mine.

v6

In react-router-dom version 6 all routes/links are always exactly matched. The end prop of the NavLink has more to do with the trailing slash "/" than it does route matching.

If the end prop is used, it will ensure this component isn't matched as "active" when its descendant paths are matched. For example, to render a link that is only active at the website root and not any other URLs, you can use:

<NavLink to="/" end>
  Home
</NavLink>

To address your question

Why React Router NavLink prop exact (v5) vs. end (v6) lead to different result with trailing slash in url

This is because you are comparing apples to oranges. If you want to achieve the same link behavior from v5 to v6 regarding the trailing slash "/" then omit the end prop.

<NavLink
  to="/notes"
  className={(navData) =>
    navData.isActive
      ? "bg-indigo-50 border-indigo-500 text-indigo-700 block pl-3 pr-4 py-2 border-l-4 text-base font-medium"
      : "border-transparent text-gray-600 hover:bg-gray-50 hover:border-gray-300 hover:text-gray-800 block pl-3 pr-4 py-2 border-l-4 text-base font-medium"
  }
>
  Home
</NavLink>