/kill @e Destroys paintings and armour stands.

Thank you for your report!
However, this issue is Working as Intended.

Paintings and Armor Stands are entities. The command /kill @e kills every entity within your render distance; thus this behavior is intended.

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An "entity" is a mob (in the case of Minecraft), unless there's some other definition documented somewhere? I don't think its unreasonable to expect one's paintings and armor stands to remain after you issue a command to kill mobs.

What else falls under the "entity" classification? Is this documented anywhere for us to review prior to using commands such as /kill?

Note that I work with Java edition almost exclusively, so my definitions may be slightly off.

In the case of Minecraft, an "entity" is anything that is in the world that isn't a block. (There are block entities, but those are their own deal and I believe PE doesn't distinguish them as much). Things like minecarts and boats, items, projectiles, primed TNT (but not the TNT block), falling blocks (e.g. sand, while it's falling), and leash knots are all entities. Item frames are entities in Java edition, but are, apparently, blocks in PE.

The phrase "mob" is, for whatever reason, reserved for living creatures in the world. Mobs are a subset of entities. Yes, this distinction is a bit confusing.

There is a list of entities at this wiki page; it mostly talks about the Java edition but I believe that Bedrock shouldn't be too far off.