Ah, got it now, thanks Daniele. Sounds like an exciting problem.

Could clients potentially do some work on the posting side too? For example when posting a reply there’s a checkbox marked “non-follower friendly” or something to that meaning, and if you tick that then when you write it’ll perform some opt-in action that’ll make life easier for the clients of lurkers and other non-followers to connect the dots and surface your reply when they browse the thread? 

Or a popup for the same, with options for "always", "never" and "ask every time" that can be adjusted later in settings. Other words push some responsibility to the posting side, make the math easier on the deducing side.

If yes I’m guessing there'd also be many thread-joiners who would be more than happy for their replies to be less visible to lurkers and other non-followers, glad to be given the choice but would choose the opposite: opt out, purposely keep things so thread-group clients can easily sniff out the reply but non-follower clients, even those doing their best to follow the traditional outbox paradigm, will have a hard time sniffing it out (as you say is the case now). Becomes like a feature, not a shortcoming.

Also if yes then I wonder what such an opt-in action at time of posting might be?