As someone who moved away from TER for quite some time, I can tell you why these boards are highly inactive:

1. TER being unwilling to take down fake reviews is a big deal and enough to make people want to delist (its one of the reasons I originally delisted, not to mention when I delisted in 2018, I hadn't even had a review in over a year).  

2. TER shut down for American providers for well over a year after FOSTA/SESTA got passed, making it difficult for anyone to participate.

3. Social media exists, which not only circumvents the demand for reviews (you can find out all about a provider, their interests/niches/etc).

So, why did i re-list, you ask? TER sends web traffic, despite the boards being dead. Participating is pretty rough to do when either no one responds to posted threads, or the only threads posted are really specific ISOs/same-day requests that providers don't cater to/fit the bill for.  

The industry is VERY MUCH alive and well, you just need to head over to Twitter to see all that.