DIN-compliant refers to switch and keyboard designs that comply with German workplace ergonomics standards believed to have been introduced in the early 1980s.
DIN standardisation required keyboards to be comparatively low profile. One major change in switch design was to bring the mounting plate much closer to the PCB, so that the keycap could move down past the point where the plate used to be in earlier designs. It is assumed that Cherry MX and Alps SKCL/SKCM series were designed specifically to meet these standards, as they both feature adaptations to allow the keycap to move down around the switch.
Some switches are confirmed to have been specifically designed for German standardisation:
The following diagram illustrates the change in design. Note that Alps SKCC series/General Instrument Series S950 is almost the same height as the successor Alps SKCL/SKCM series, but SKCL/SKCM positions the mounting plate much lower down. As such, the keyboard profile can be significantly lower.