I think what you are talking about is instances that may have a large population of marginalized groups, and the fear that someone is creating a database that could be used to easily seek them out and use it for trolling and such. Which I think is a very valid concern.
And as mentioned above, you have the crowd that wants to take an instance and give all their posts over to for-profit corporations like Threads and Bluesky, that should not even be called part of the fediverse IMHO.
I don’t know how you make a global search for the fediverse that avoids both of those issues though.
I think everyone understands that what they type is public, but there is a difference between posting something to a community that may be small in nature and would likely only be found and read by those also interested or a part of said community, and someone creating a database of ALL this type of content that some troll can use to more easily target people and blast their hate filled replies out to.
I don’t think anyone is telling groups “post to my instance and you’ll be safe from threads”. But when people want to do things like creating bridges from Threads to Lemmy they strip admins of the ability to block Threads content from Lemmy instances that defederated Threads for a reason.
These topics have far reaching implications that just “I want all the search!” and anyone that doesn’t want it is holding the fediverse back.