Enable persistent player deployables which don't vanish when players leave or disconnect. The server still keeps track of individual player deployables so this can't be used to deploy infinite ammo bags or anything, it's just a quick fix to an annoying "feature" that leaves you at a disadvantage if someone disconnects.
Doesn't apply to sentry guns by default, but you can change that in options if you want.
Update r2:
Playerdrop will now keep track of player deployable counts when clients leave or disconnect. If a player still has remaining deployables, they will not be reset, but instead keep that number of deployables upon rejoining. It'll look like they have them all due to limitations in what can be sync'd, but they will only be able to deploy the amount they left with.