> I think newer versions of tar already have support for Zstd.

You don't really need to have support for a particular compression fromat in the tar command since the tar format does not support compression at all and instead the output of tar is passed through a compressor such as gzip or xz that has its own encapsulation format. All the support in tar does is add switch to start that (de)compressor for you and, for GNU tar, to detect it based on the file extensions.

a tar.something is just a tar archive passed through a compression filter like zstd or gzip. Anyway, tar needs metadata extensions in order to allow random access; most tarballs you'll find don't. So you need to read 5TB of data to find the file at point 5TB in an archive of size 10TB. Which is why as a general purpose archiving format, tar needs to be applied to use cases where it works. It gets old really quick if you don't have random access and random access requirements, plus, remediated by SSDs these days for a bit, it's really time consuming.