https://news.ycombinator.com/newswelcome.html.
https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...
I don't think this particular issue is related to new younger audiences, because—by inspection at least—the initial wave of objections isn't noticeably coming from newer users. I haven't specifically analyzed that though.
Complaints about "notable" drops in quality go back all the way to the beginning of HN. Of course it's possible that they're all true, but that would be a lot of notable drops. Or maybe some are true and some aren't, in which case we're fluctuating within a range... or maybe they didn't use to be true but now they are true? That's of course what the person perceiving the "notable drop" always feels—but there are also other explanations for this feeling, such as sample bias and nostalgia bias (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...). It's pretty hard to establish anything objective about it. If I randomly look at past frontpages and past comments, they don't seem consistently better or worse (but see the previous sentence).
I think HN is suffering from its own success with a notable drop is comment / post quality.
The first rule of HN is you don't talk about HN. ... also, if you've found a new forum, please let me know!