You can define a metric such that future data doesn't affect past data. Here's a straightforward one: a user is inactive at time t if they haven't posted in the period between t and t - k where k some constant time period one picks. So let's say k is a year and you're looking at active users per year†. So in your last example, the user would be counted as active in 2007 and 2008, counted inactive in 2009 to 2022 and would count as active in 2023. If you truncate the data at 2022 nothing changes.

† year is probably too big of a window for this (I'd take something like a month) but let's stick with it for now