I've recently started working through AI safety posts written on LessWrong 1-3 years ago; in doing so I occasionally have questions/comments about the material. Is it considered good practice/in line with LW norms to write these as comments on the original, old posts? One hand I can see why "necro-ing" old posts would be frowned on, but I'm not sure where else to bring it up. You can look at my comment history for examples of what I mean (before I realized it might not be a good idea)
There are two categories of posts (for this purpose): Some are comments or reactions to something going on in the rationalsphere, or relatively lightweight explorations of a topic.
Some are summaries or exploration of topics that will remain useful for many years.
LessWrong is doing great work in bringing to the forefront the latter ones - sequence posts, highly-useful commentary, etc. Relevant comments and questions only add value to these posts - they make them better next time they get referenced, and they add a little signal that someone finds them relevant today.
For the more ephemeral topics, there's probably not much harm in commenting, but you may not get any answers.
Nope. That's part of the value in commenting/voting on old posts. You're mostly going to care enough to do so on the ones that seem relevant to you, which are the ones that might be worthwhile for others to look at.