no, it's really the user interface of specific implementations. one could modify the code running an instance to keep track of viewed posts and refrain from showing them again. an app could do that too, even if the instance kept on informing it of boosts.
ActivityPub will indeed flag boosts as you describe, but that doesn't have to carry over all the way to the user interface
It is not just the user interface, it is the very basic design of the fediverse.
One of the annoying features is that the same post shows up again and again in my feed, every time someone I follow boosts it. The old USENET avoided that by keeping track - in my local machine - of the messages I had already seen. Like any email server does. But the fediverse cannot do that, because it cannot keep such memory anywhere...
In this paper i searched for - peertube - castopod - friendica - misskey - calkey
Not one occurance.
The fediverse is in first reading a protocol, which connects all the different services.
And yes, it's work in progress.
If you are looking for one polished interface... living in front of one service owned by one billionaire... you are wrong in fediverse.
But... if you never tried the other services from this network, if you ignore it constantly and only look on mastodon... write about mastodon... a service, that ignores standards from the fediverse... (i call mastodon "the microsoft of the fediverse") then you don't know the network and it's magic.
Yes, there can be some kind of polishing in thf protocol, on some of the services, some UI/UX... but it is free from surveillance-capitalistic billionaire-owned companies, which only try to show you MORE ADVERTISINGS.
That is one of the problems of the fediverse: its is fragmented into many more or less incompatible platforms. Few users have the time to check out more than one, or migrate between them.
The fediverse itself is a half-baked idea, poorly implemented. The old USENET worked because it had a much better overall design.
I can see that an app could keep track of the messages I have read (like the old mail and Usenet readers did). Or a centralized service like Xitter and Whatzap. But how could a browser-based AND decentralized service do it?
if you will delegate to an instance the decision on what to show you on your browser when you connect to it, you'd have to pick an instance that does that for you. mastodon mainline could make that configurable. someone could patch mastodon in their instance to behave as you wish. you could run your own instance. you could run something other than mastodon that behaved as you wish. these are all in line with the notion of decentralization and browser-based UX. I sense there's some other unstated requirement.