Reactions on issues and PRs are helpful to gauge their importance.
For example, the list of issues sorted by 
reactions shows which issues people find relevant.
While that’s not the only metric to determine acceptance and prioritization of a change, it can help move important stuff further ahead.
On the nix repository, I found this little detail: All PRs have a footer that encourages people to leave a reaction to show their interest in the topic.
See this PR for an example. This is part of the PR template on the repo.
I’m wondering if we might want to try this in the crystal repo as well?
It’s not super important. Reactions work just fine without this encouragement.
But it’s less organized and can be messy considering there are a number of different reactions with different meanings.
Maybe such a note could be useful. It reminds readers about leaving a reaction. And it points out the reaction as a single metric.