Do I need to treat shiplap panels?
Most fencing material comes pressure treated as supplied. This prevents the wood rotting for an extended period of time.
New fencing and indeed garden sheds need painting with a product like Fencelife as soon as possible to protect it from the elements mainly frost, rain and especially sun.
Without this protection the timber can shrink, warp or split.
You also get to choose a colour rather than the original treatment colour of green or brown.