So, if you necropost, people tell you “Don't necropost! Create a new topic!”
Then you create your own topic and people tell you “Duplicate https//scratch.mit.edu/discuss/topicyouweretoldnottoposton”

How are you supposed to do anything on the forums when this is the case almost every time?

Well necroposting is easy to avoid, and you can always search to see if people have already made the same suggestion as you

A post is only considered a necropost when the topic's been dead for 3+ months, and the post isn't (that) constructive, so lets say there is a suggestion for Cloud Variables (I know this is already implemented, I'm just making a point)

Cloud Variables Suggestion from 2011 wrote:

I suggest adding server-stored variables, or cloud variables, so support?

A Necropost wrote:

support!

A Constructive but old post wrote:

Support, but there should be certain limits, like 256 characters per variable and 10 cloud variables per project

As long as you are constructive, I don't think that Necroposting applies to the suggestion forums. If there is an old suggestion, that hasn't been implemented, and you leave a constructive post saying why you support or don't support, it's fine. The problem with Necroposting is in other forums such as Questions about Scratch, when the question is resolved, but people still post; or when posts are unconstructive, like thread hijacking. So if you see an old suggestion which you support, feel free to say support and your reasons why.