Question and Answer count
Thanks @user620588. The number of questions that are asked versus the number of questions are answered may indeed make a metric to determine the site health.
That graph shows a steady decline in the absolute number of questions asked and answered on ServerFault.
So by that metric the site is in decline and has been for years.
2020 saw a trend reversal, but only in the number of questions asked rising sharply.
(Maybe because people working from home due to restrictions get less mentoring and/or are less inclined to "bother" their colleagues to ask a question and turn to online resources instead?)
The absolute number of answered questions in 2020 didn't show any increase at all, but rather continued the downward trend. So percentage-wise much more questions remain unanswered now. That aggravates the appearance of decline of the site.
Those numbers suggest more sand and fewer pearls...
Since you need to be registered user to be able to vote the number of votes cast might be another metric to show “community” and participation :
That metric shows a different pattern, at first glance despite some fluctuations the number of votes in the largest categories appears to remain quite steady.
That suggests that the number of additional questions posted in 2020 did not result in an increase of the actively participating community, but it doesn't suggest a decline either.