The answer is yes-ish to all questions.
EBT is not exactly losing members but its active member growth has almost stalled (I guess). Also, an average of 9 new members every day has been steady in the past three years.
Notes/day is in slow decline in the past nine years. Lithuania and Estonia are the happy exceptions.
Because there are less notes being tracked (but more notes being printed), the hit ratios (both per day and per notes) has been declining too. Some hits are more amazing than ever, though, both in terms of time interval and distance.
There's no study, that I know of, of the popularity of banknote tracking or Euro tracking. It comes and goes depending on the media. In the past 8 years or so, fewer and less active EBTors have been involved in communication with the media, and that could be due just to random (lack of) luck.
At the same time, almost every Working Group of voluntary EBTors has been working a lot less than up to 2009 or so, which means less new features to keep everybody happy, and updates less frequent.
You may want to check these stats:
https://eurobilltracker.com/stats
https://eurobilltracker.com/notes/?command=8&tab=1
As to the meetings, we're back to one international meeting per year instead of one Winter and one Summer meeting that was being held up to two years ago.