I recently found one of my supply units (provisioner) at another settlement that means they apparently travel the wastes. Does this mean that they can actually die if they run into some enemies while I'm hanging around? Should I be equipping them with better gear?
Your settlers at home can die. Note that once a 'settler' has been assigned to a supply route, they will be a 'provisioner' the next time they are loaded.
Provisioners who are away from a settlement can't die. The can get knocked down for a few seconds like other essential NPCs, but they get right back up. Currently, the closest I've observed this is at the Rotten Landfill (east and slightly north of the drive in).
It may be that:
a) Provisioners are just essential always
b) Provisioners who are outside a certain range of a settlement are essential, but can be killed when close to it.
or c) Provisioners who are outside a certain range of their home settlement and/or destination are essential, but can be killed when close to them.
Clearly, more testing is required, but given what I know about how the essential flag works, it's a pretty safe bet that provisioners are just essential all the time, and therefore can't be killed.
No, settlers can't be killed by raiders or by anyone apart from you. Apart from the case if you failed to defending quest. They only knee like companions. But you can kill them, and it can happen accidentally during fighting so don't ever use grenades if your settlers are near, they have very low health and also better always to use vats in settlements.
Settlers are flagged as protected and can not be killed by NPC's. Period. It doesn't matter if they're in a settlement or travelling as a provisioner. They can technically "die" if you fail to protect a settlement but this isn't because an NPC killed them it's because when you lost the battle, the game despawned one or more of your settlers. The people claiming their settlers were killed by raiders or deathclaw are either flat out lying or accidentally killed their own settlers and didn't notice.
See this video for proof:
Up untill recently any provisioner (settler that takes care of the supply line) that was attacked and went down got up again a bit later (same as followers). But a while ago I threw a molotov in a group of ferals and it made some cars explode. Just when trowing the molotov I heard someone speaking. It seems a provisioner was just passing by, and yep, he died from the explosion. So they can die, maybe only from really high damage? Did not check if supply line disappeared. I had a savefile, reloaded, and killed ferals with weapon. Provisioner survived and moved on.