Using Discourse at these two sites:
Until a recent upgrade, there used to be a small, rectangular mid-grey button at bottom right of the page displaying New topics (from the ‘New’ tab at top) with the label ‘Dismiss’ or ‘Dismiss All’.
It seems to have gone in this version.
It was particularly useful when not every thread was relevant or needed to be attended to.
Now it’s necessary to click into each thread - actually twice - for them each no longer to show up in one’s ‘New’ list.
Good luck with fixing this - and thanks!