This was brought up in the NormConf Slack the other day. There, an article was mentioned about the cultural reasons why Mastodon’s search works the way it does (https://midrange.tedium.co/issues/how-mastodon-search-works/).
TL;DR Mastodon is a reaction to Twitter and has different norms for engagement. Maybe global search is incompatible with that.
As an experiment I implemented a simple Google CSE that uses a domain list of all known English Mastodon instances as listed on https://instances.social but it doesn’t seem to work very well: https://searchmastodon.social
What does HN think?