What does 乃 mean here?
Here "乃" means "be". This sentence means "This area is for old posts, and these posts are read-only."

Next time, if you find "此乃" these two characters together or a noun+乃 together, "乃" properly means "be". For example:


  此乃我的书.
   This is my book.
  
  意识乃是大脑的一种自然属性。
   Consciousness is an emergent property of the brain.