I "might" have uncovered what this diode is doing, acting as an OR-ing diode in conjunction with one in the BCM that has its own voltage sensing to detect a pull down, these two diodes isolating each branch, to sense when the trunk is open, since the trunk latch switch ("might".... probably?) serves double duty for sensing the trunk is open, and to turn the bulb on.
This is not a schematic specific to that vehicle, but shows the principle. Even if you elect to switch to using an LED bulb, I would still replace the diode, though technically yes, a little festoon type LED retrofit bulb is just going to have a few LEDs in series and will do the job of the separate diode, but will have to be installed with the right polarity. I wouldn't even chance it though, better to put it back to OEM state with the diode included even if you swap in an LED bulb, so at any time in the future, any tech or owner won't have to guess what has been cobbled together and how to resolve any issues.
LED retrofit bulbs don't always have long lifespans, often not as long as the incan bulb they replace because they are driven too hard and heat up too much, so it's entirely conceivable that at some point in the future, someone might try putting an incan bulb back in to replace a failed LED bulb.
View attachment 133328