I feel like twitter has been around long enough that people that want to use a platform like it will be aware of the concept of a hashtag for finding posts on a topic, etc.
Twitter/microblogging is a weird thing in general but you seem to have the idea. It can be useful for following events or people who write interesting things.
Mastodon hashtags are the way to discover conversations and people to follow, they go across instances in search.
It was named Reddit Sync before Reddit forced apps to rename a few years back for trade mark reasons.
Sync was named for a feature it had where you could sync posts to your device for offline use. It was super useful as recently as a few months ago when I wanted to mindlessly scroll on a plane.
I think fighting this will be a mistake. Instances ran by the likes of Tumblr and Meta can only bring more people into the fediverse, and when they're in it will be easier for them to move around.
The great thing about AcitivityPub is it lets the people who want to be in larger more centralised servers connect to those who don't fairly seamlessly.
You don't need to do anything when you get a security advisory if you're not impacted or the risk is acceptable. They're just there to advise you.