I've only skimmed the thread as it's quite long, but basically @nathanKmcwilliams larger post a short ways up encapsulates my feelings as well, as well as @joannekwan 's reply shortly after as well as the sentiment that @milejdyvan has been offering throughout-
In short, while implementing a feature to help remove dead subs would indeed most likely be useful to some degree, I feel like simply striving to make better content that people want to engage with is a better route for the average author rather than stressing about how many dead subs you have.
Like it's impossible to get actual data on this without knowing the exact ratios that are taken into consideration, but I find it hard to believe that a normal ratio of dead subs is really sinking anyone's ship that hard, you know? As Joanne said, subbing to a series to return and check out later is typical reader behavior on these sorts of sites- it's unlikely that anyone is getting singled out and getting a much higher amount of dead subs than usual (unless they're doing sub for sub which, like, don't do that lol).
Even if you were to remove all of the dead subs, that still leaves you with whatever typical engagement numbers you already have plus all of your silent/lurking readers which represents the majority of readers. That might help a series' trending performance a little bit but the series will still be getting consistently beat out by series that are able to entice more readers into engaging more frequently.
This quote in particular stands out:
Algorithms are fickle and subject to change. It's of course good not to go too far against the grain (i.e. instead of saying 'algorithm be damned, I'm gonna sub for sub anyways!' it is sensible to avoid doing that to reduce the amount of unnecessary dead subs on top of the typical amount), but rather than worry about playing to it too much... just work on improving your craft imo. I mean, that'll happen either way right? But having "oh jeez, I have so many dead subs!" weighing on your mind is just an extra level of stress that's unnecessary.