Function of Pin L2 in Max10 U169 package (10M16SAU169)?

Unfortunately I can't tell you what the "D" in Pin Planner exactly means, but MAX10 has no "Dual Purpose Pins" like other FPGAs because it doesn't use external configuration memory.

Your first source of information about pin functions should be the Pinout File of the respective device, it shows that the said pins are regular IO pins without optional function.

I don't understand why you would connect the pins to a supply rail? According to the default configuration of unused pins as tri-stated with weak pull-up, you'll leave it unconnected.