>Just wanted to know if there's something I should have known but didn't, about statcounter and generally such add-ons which require a http call from the webpages to work. I confess I was slow on the uptake - the decline in counted visits began gradually in 2017, and has now dwindled to near nothing, and I ascribed that to google's ever changing rules (which did happen, but were they the cause?).

Weeeellll,
discussion about that branches out into cross site scripting and - tracing with black hats.
Includes web bugs to trace or build profiles in biz-friendly grey hats.

NoScript something your setup works with, but might stop counter calls.
PiHole something which will have domains like statcounter in default black lists.

Defensive setups becoming new normal is my guess.