The Docusaurus tabs use MDX, it's not a custom syntax for tabs, it's a powerful low-level way to use React or frontend interactive components in markdown, and likely to become more and more widespread (so hopefully portable and yet very powerful)

It's also possible to implement tabs with Rehype syntax in Docusaurus in user land thanks to a Remark plugin, so portability remains possible but not free.

At the end of the day, md is not the most powerful out of the box and it's difficult to ship something meaningful without any extension