In the interests of clarity, I'm going to answer based on the premise that Thread Necromancy is inherently bad. If it's resuming an existing discussion with no negative impact, then it's not really Thread Necromancy. If none of the three factors below apply? Then I woudn't call it Necro.
That being the case, I think the issues with it are:
1. You're responding to an old discussion that the participants may no longer be interested in or hold the same perspectives on. If it was a contentious one, getting notifications about an old argument you had months or years ago may be annoying and unpleasant.
2. Rather than starting a fresh discussion of the topic, you're "making" folks wade through some (potentially large) number of old posts to get "caught up" on the discussion, and again, some of those participants may no longer hold the same perspectives, be on the forum at all, or even still be alive in some cases! Maybe I posted a reply on page 5 and in the process of reading through, not noticing the dates, you reply to someone's comment on page 2 and it's from 2009. That tends to be a waste of your time.
3. The context of the discussion may have changed in a way that makes some (potentially large) portion of it irrelevant, such as if there's a rules debate involved and a newer ruling has come out. Or if there was a big discussion over the best way to build a character given some constraints in the materials published at that time and, say, the publication of new options in Tasha's (for example) removed those constraints.
So I think there's no number or specific age which makes it Necro, but it's really a question of whether the thread has been dead long enough that resurrecting is liable to cause confusion in other readers or where a fresh post would be more accessible in general. In the latter case, if you think there is still useful context from an old thread, it can often be a good idea to mention it in your OP of a new post and link to the old one.
