Usenet died for a number of reasons, and those reasons are core to what has kept any revival of NNTP-based forums, or strongly-similar variants / derivatives, from succeeding.

I've given my own take on this (just one random space alien cat's take, some familiarity, no major at-the-coalface experience) in a piece that's seen some currency, "Why Usenet Died", posted at ... Reddit:

https://old.reddit.com/r/dredmorbius/comments/3c3xyu/why_use...

The TL;DR:

1 It got spammed to death.

2. It lost control over its culture, and that culture was crucial to its functioning.

3. It was too problematic for ISPs (or others) to provide ready access to it: spam, harassment, child pornography, and copyright violations all posed massive concerns.

4. There was no viable business model for providing the service.

Beyond that:

There has been a Usnet 2.0, or rather, Usenet II

The original Usenet was small and had very limited access. As of April, 1988, there were 381 newsgroups, 57,979 messages, an estimated 141,000 readers, of a total user population on connected hosts of 880,000, via Brian Reid's "USENET Readership Summary Reports". Virtually all were at selective-admissions universities, tech companies, and a small selection of government agencies.

(In many ways, 1980s Usenet was the equivalent of edu-only Facebook. Think through that a few times.)

Usenet had many useful features ... and failings. Much of Usenet's decentralisation and client-independence meant that bad nodes, general adversaries, and flouting of tradition (the Eternal September, spammers, AOL, ...) could destroy it.

Usenet had little to no archival, filtering, or spam protections. There were attempts to bolt on solutions. Those ... fared poorly in general, either in effectiveness or unintended consequences.

In spirit and role, the closest thing to Usenet for the moment is probably Reddit. For anyone interested in my views on Reddit, see the recent and pinned posts in https://old.reddit/r/dredmorbius

Much of Usenet's early success was a direct result of its institutional foundation: selective-admissions, computer-science-heavy research universities. Those had a group-communications problem, and could support the infrastructure for a solution, but couldn't scale appreciably beyond the million or so member community of late 1980s / early 1990s Usenet. Today's Internet is global, is non-selective, has no common culture (other than, perhaps, advertising and other forms of behavioural manipulation), and no domimant institution (with the same exception). This bodes poorly for re-emergence of any Usenet-type mechanism.

Another chief problem is that static group forums have their own strengths and weaknesses, but among those is overhead and latency. This is exhibited on Usenet itself in the huge land-rush of newsgroups defined when world+dog decided that they had to be there ... most of which turned into unmoderated and unused wastelands, either flooded with spam or entirely empty. Slapping labels on room plaques does not a community make.

You can still spin up your own limited-access NNTP server, and run email and Web-based gateways. I've been active on the Fediverse for over a decade (Diaspora, Mastodon), and though there's some similarities, and considerable shared individuals, the scope is not the same, and group discussion in particular is ... not well-facilitated. (There are bolt-ons, e.g., gup.pe on Mastodon.)

Email lists and IRC (which does* support pretty flexible temporary channel creation) are other options, both of which afford control and free-flowing forum characteristics.