Partial answer:
I have a different contender:
The LES-5 satellite, launched in 1967, was an early experiment in satellite-based communications broadcasting continuously since it was launched in 1967. It was decommissioned and placed in a “graveyard orbit” in 1972.
https://www.popularmechanics.com/space/satellites/a32293223/les-5-satellite/
It became notable for starting to transmit again after 53 years of being 'dead'.
But this paper says 1977:
The geostationary ring is a valuable resource currently populated by more than 340 operational satellites. Unlike in low Earth orbit there is no atmospheric drag which will remove abandoned objects over time. Therefore, it is the responsibility of the spacecraft operators to keep this unique orbital region clean. Already in 1977, Perek (1977) proposed1 that spacecraft should be systematically removed from their geostationary orbit (GEO) at end-of-mission. In the same year INTELSAT sent for the first time in space history an aging satellite into a GEO graveyard orbit.
Since then a number of guidelines and recommendations were issued by national and international institutions...
1Perek, L., Physics, uses and regulation of GSO, IAF-SL-77-44, 28th Congress of the Int. Astronautical Federation, Prague, Czech., 1977
The paper is from 2005, and at that time eight Intelsats had already been boosted to the graveyard orbit.
Unfortunately the document does not state which satellite was sent in 1977 and by that time there were numerous Intelsats that were retired.
However:
- Intelsat III F-2 ceased operations after only a year and a half in orbit, in mid-1971. It is listed as in supersynchronous orbit.
- Intelsat III F-3 has a wiki (sorry) entry:
At the end of its service life, Intelsat III F-3 was raised into a higher orbit to reduce the probability of it colliding with an operational spacecraft.
Although it was decommissioned in 1979, two years later, it could be said that in 1977 while it was still serviceable though not in service, that this was when it was transferred to its graveyard orbit. (No actual sources atm, but its the only one to get noted for the orbital shift)
Including all of the first, second, and third generation satellites that were launched and retired and/or failed before or in 1977, that makes 8 potential satellites to have been the first.