T285293#7311910 suggests that https://wikidata-federated-properties.wmflabs.org/ is meant to demonstrate the current state of Wikibase Federated Properties v2, and I wanted to link to it in a discussion on the topic, however the server is on PHP 7.2; the current version of MediaWiki requires PHP 7.4+ and so an error page is displayed:

MediaWiki 1.43 internal error
MediaWiki 1.43 requires PHP 7.4.3 or higher; you are using PHP 7.2.31.

Meanwhile, in https://wikidata-federated-properties.wmflabs.org/w/api.php?action=wbformatentities&ids=P1|http://wikidata.beta.wmflabs.org/entity/P694 from T287019:

Parse error: syntax error, unexpected '=' in /var/www/html/includes/Setup.php on line 150

This is due to use of the null coalesce equal operator in PHP 7.4 in the code $wgAutoloadClasses ??= [];

The project is currently without a team so I am not sure who is responsible for this server. Many involved with it appear to no longer be involved with WMDE, @Tarrow being an exception.