Sorry for that may be i should have added screen shots n the earlier comment itself                                                    I am inferring based on the links inside https://docs.databricks.com/aws/en/release-notes/runtime/

and inside 16.4 i see

In summary 

Though you see in release notes

official Parquet zstd compression support is recognized starting with DBR 8.0 in later product documentation, hence using zstd with DBR 7.x remains unofficial and MAY carry risk.

The most definitive check is to perform dedicated testing on DBR 7.3 LTS clusters by writing and reading Parquet files compressed with zstd. Watch for job errors, unreadable file errors, or degraded performance . Hope this helps