ZStandard compression was added in MySQL 8.0 but is currently not supported by MariaDB Connector/C.

MySQL indicates ZStandard compression support by setting CLIENT_ZSTD_COMPRESSION flag (1UL << 26).

For using ZStandard compression the client set's CLIENT_ZSTD_COMPRESSION flag, instead of CLIENT_COMPRESS (=zlib compression) flag.
In addition to the capability flag, the client will send a 4 byte (unsigned integer) after connection attributes which specifies the compress level:

Client Handshake Response:

...

if (server_capabilities & CLIENT_CONNECT_ATTRS)

    int<lenenc> size of connection attributes

    while packet has remaining data

        string<lenenc> key

        string<lenenc> value

*NEW* 

if (server_capabilities & CLIENT_ZSTD_COMPRESSION)

    int<4> compression level