Lotus, WordPerfect, Harvard Graphics; most DOS and early W3 apps had them. They were cardboard strips that fit around the row of function keys, or rested on the open keyboard space above the F keys. (Keyboards use to have substantial space between the F keys and the trailing edge, as much as an inch and a half.) The templates indicated what each key did in that application. The most complicated ones had several rows - what the key did by itself, what it did when combined with ALT, with CTRL, etc. When you switched programs, you switched templates.

You used to be able to order blank keys and label your own. They may still be out there; I haven't had a reason to shop for them in years.