I can't get a through hole diode to work on mechanical keyboard PCB key
Those look to be standard 1N4148 or 1N4448 “Silicon axial signal diodes . Almost any signal diode will do as they serve to select the switch during  a row or column scanning.

use the Ohm mode across the switch terminaLs to confirm. if it is an often used key , compare the mechanical feel and check the diode solder joints as well. Do all these with power disconnected.

Normally Diode test mode is best way to confirm which uses a small current source to get 0.6 to 0.7 is forward bias (ON) with red on Anode and black on cathode (bar) with OFF being 1xx over scale for reverse voltage.  Not exactly the same as resistance measurements but as long as they show high R in reverse and low R &lt;&lt;100 in forward, they are probably OK. And not &lt;10 ohms in both polarities.