Supported?

If you check the plugin’s development tab you can find links to the source repository and it does indeed look like it’s in active development.

Not a whole lot has changed, but they’ve updated the readme.txt to include support for WordPress 4.8.1: https://plugins.trac.www.ads-software.com/changeset/1730766/hyperdb/trunk/readme.txt

HyperDB is under active development and still used on WordPress.com and other large WordPress sites. There was a problem which prevented the plugin page from being updated, but it’s now fixed and you can find the latest version, 1.4, available for download there.