Broken or just no longer available? I guess the latter. Has it been available in the past? It's the first time I hear about it.
The opensearch plugin I've found was released for Elgg 1.7 (https://elgg.org/plugins/523102). This plugin would no longer work on Elgg 2.0 (used on the community) and therefore it can't be used anymore if there's no updated version available elsewhere.
The community is indeed using the version meant for Elgg 1.8: https://github.com/cash/opensearch, which is not compatible with Elgg 2 and is therefore not working anymore.
The plugin hasn't been updated in a long time, so the plugin author (Cash Costello) may not have time or interest to maintain it anymore. You could perhaps ask if he could move the project ownership to you in Github, so you could continue his work. (Assuming you are a developer, of course.)
Let me know if you're interested, and I can also transfer the ownership of the community plugin project (https://elgg.org/plugins/523102) to you.
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