I am trying to remember the deal on this. My 1991 Deville had the yellow button on the front edge of the trunk opening. I think this was standard for cars without remote keyless entry, or without the factory security system or something (neither of which were present on my 1991). It was really just a convenience so you could unlock the doors from the trunk when it was open, after putting packages in so you didn't have to use the key to open the door(s).
With all Cadillacs since at least 96 having RKE, this was a redundant feature that was eliminated. Funny, I only recall seeing it on the C-bodied 'lacs, the D platform (Fleetwood [Brougham]) didn't AFAIK and I don't know about the Eldo and Seville lines as I have not seen much of the pre-96 in these models.
Anyway, it is another Cadillac curiosity of the past, like the color matched & carpeted front passenger side "trash can/umbrella holder" and rear deck storage compartment on the C-bodies of yore.