Why has X11 system clipboard interoperability changed from Emacs 24 to 25?

[itchybeard <itchybeard@protonmail.com>]

In the Emacs 24 GUI for X11-based desktops I can set
`'(x-select-enable-clipboard nil)` in my config to prevent standard kill ring
commands like `yank`, `kill-region`, and `kill-ring-save` from affecting the X11
system clipboard, and it still allows me to explicitly interoperate with that
clipboard by invoking the commands `clipboard-yank`, `clipboard-kill-region`,
and `clipboard-kill-ring-save`.

But in Emacs 25 the variable `x-select-enable-clipboard` is replaced by
`select-enable-clipboard` which when set to `nil` in my config yields different
results: *all* interoperability with the X11 system clipboard is disabled. Even
the commands `clipboard-yank`, `clipboard-kill-region`, and
`clipboard-kill-ring-save` revert to using Emacs' kill ring exclusively, in fact
they have identical effects to using plain `yank`, `kill-region`, and
`kill-ring-save`, so what's their purpose when `select-enable-clipboard` is set
to `nil`?

What is the reasoning behind this change in behavior, and is it possible to
replicate the Emacs 24 behavior in Emacs 25 using built-in configuration
options, or will I have to start using a custom solution like simpleclip? I want
a kill ring and X11 system clipboard that are independent, and I want
interoperability between them but only explicitly, e.g. to use `yank` to paste
content from the kill ring and `clipboard-yank` to paste content from the X11
system clipboard.

There is an open bug related to this changed clipboard behavior:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27442