I found out about that a few years ago.

If you run "openssl ecparam -list_curves" on a RHEL clone, you will only see p-256, p-384, and e-521 (and e-521 was only added in v7).

If you build libressl and run the same command, there are dozens (and Canada doesn't allow software patents, so it's legal there).

On OpenBSD 7.1, I see this result:

  $ openssl ecparam -list_curves | wc -l
  102

Note the 2 Oakley curves take up 8 lines. There are also 17 matches in this output for NIST curves.