This is a picture of a premium wood sash with true wood muntins and individual 12×12 insulated glass panes.
Known generically as a “true divided lite insulated glass window”. The glass is held in place with wood glazing sticks. (NO putty!!)
With proper maintenance these windows with their all wood construction should easily have a service life of 100+ years.
But the devil is in the details.
As it happens insulated glass panes have a relatively short service life before the seal between the insulated panes is compromised and moisture accumulates and randomly fogs the window. In some cases less than 20 years.
It is an exercise in futility to replace one 20 year insulated glass pane with another 20 year insulated glass pane in a long life sash so the owner opted to use single sheet 3/8 inch plate glass to exactly fill the space formerly occupied by the insulated glass pane.
As an added bonus the plate glass replacement is cheaper than the insulated pane counterpart.