I have found some katanas in the lower depths of the mines from a group of ninja zombies near some titanium nodes. The ninja zombies have random loot so you may unfortunately only find ninja shirt, ninja pants, and ninja hood, but not have any katanas drop.

as to why it is not in the spawn tables, it may be a map custom katana that has a different ID, like how cloth and scrap metal are map custom items with different IDs than vanilla Unturned cloth and metal scrap. Other wise IDK.

Sorry I can't give directions, with how mazey and same looking the mines are. I would suggest head lamp + oxygen mask + oxygen tank + two or three "bottled air" to go that far into the mines. Head lamp, oxygen mask, oxygen tank, and bottled air can all be found in the upper levels of the mines around where the mine cart tracks are. Head lamp can also be crafted with flash light + rope and bottled air can be crafted with (empty) plastic bottle + campfire. I don't know how fast the breath meter runs out without the oxygen mask + oxygen tank, but you may also be able to go to the depths with 12+ bottled air. bottled air restores 100 oxygen, even with oxygen mask + oxygen tank, so you can wait 'till you're under 10 oxygen to use it. but you lose 10 health per tick when out so I wouldn't push refilling your air too far.

I would suggest taking as few items as possible, or items you don't mind if you lost, into the depths of the mines where you can't breath, just in case you get lost, run out of air and die. It would be very hard to find your corpse and items if you got lost, and hard to get out again.

Also... in the depths of the mines... beware the [REDACTED]...

Thank you. It's a really helpful answer.