Though OEmbed may not have been a public API it would be great to get an answer on this. Several projects make reference to it, including OEmbed.com itself.
- https://github.com/iamcal/oembed/issues/437
- Wordpress: https://core.trac.wordpress.org/ticket/39410 closed their ticket because of assumed deprecation.
- Drupals builtin OEmbed functionality no longer works.
- Ghost https://ghost.org/integrations/twitch/
- Gatsby plugin https://github.com/raae/gatsby-remark-oembed/issues/129