Part of the URI standard:

      return a string consisting of the reference's path component
      appended to all but the last segment of the base URI's path (i.e.,
      excluding any characters after the right-most "/" in the base URI
      path, or excluding the entire base URI path if it does not contain
      any "/" characters

https://datatracker.ietf.org/doc/html/rfc3986#section-5.2.3

It's a client-side thing, not a server-side thing. That is, if you have a resource (like an href to another page) in your html, and that link is a relative link, the full URL that your browser ends up requesting depends on whether there is a trailing slash or not, because "relative" is relative to the current URL's directory, and what is considered the current directory differs depending on the presence of a trailing slash.