Does Pages 10.3.9 currently have a plug-in for EndNote 20?

The Clarivate EndNote site only shows compatibility with macOS 10.14, 10.15, and they acknowledge that they do not have compatibility with macOS 11 (Big Sur) at this time on M1 or Intel Macs. If you are on Big Sur, you are done with EndNote 20 at this time.

The only supported plug-in that EndNote mentions in their compatibility page is Cite While You Write for Pages, and the first paragraph requirements having been met, you can download the appropriate plug-in via this Cite While You Write page. I have no idea what version combination of Pages and EndNote that this plug-in supports, and it may or may not work. This plug-in is also as you see in the compatibility page, used for supported versions of MS Word on Mac.

Apple has a Pages EndNote Plug-in v3.2 that was intended for use with Pages v6.2 or later, and EndNote X9.3 and later. It was released in Sep 2019, and Apple has not updated the download page with explicit compatibility statements about newer versions of Pages, or EndNote. It also installs into a System library location which would be blocked on Catalina and Big Sur due to their residing on read-only filesystems. If you install this and it breaks Pages or Endnote, there is zero support here for it.