The trends tend to line up with rise and drop of subs. It's not 1:1, but it is the most accurate way to gauge that we have right now. Even the 3rd party realm census sites are only taking aggregates of armory information, and otherwise can do very little for gauging active sub numbers vs inactive.
Like I said, there is no true accurate way except Blizzard releasing internal numbers. Otherwise, even the spikes we see in this chart through google trends lines up completely with the sub increases and drops during Shadowlands.