Basically, NATO seems not to have had such plans but the Warsaw Pact definitely did.

Here I am mostly talking about the late 1970s and 1980s, the late or second Cold War era, also the era of emphasis on prolonged conventional warfare. I am also talking only about military operational plans. It was really never the desired strategy or policy of either alliance to invade the other's territory, but political contingencies could have developed and the militaries had to be ready.

Were there NATO military plans to invade Warsaw Pact territory? Apparently not, at least not at the alliance level and not using ground forces. We have not found such plans and there is reason to believe NATO forbade such planning.

During the Cold War, NATO always stated that it was a defensive alliance, but its offensive capabilities increased, in addition to which we can observe that if NATO planned to fight WWIII only on its own territory then it was somewhat hamstrung. This fundamental problem with NATO's posture was noted by numerous commentators and may have fueled Soviet suspicion.

In 1984, NATO officially adopted Follow On Forces Attack (FOFA) as part of its military strategy. So NATO was publicly confirming that, in the event of war, it would use aircraft, missiles, and other long-range weapons to attack Warsaw Pact forces who were going to "follow on" to the initial attack. FOFA would necessarily involve waging an air campaign over Warsaw Pact territory, which is where those forces would be located. At the same time, the U.S. Army was operating according to its new AirLand Battle doctrine (expressed in the 1982 edition of Field Manual 100-5 Operations) which among many other things emphasized the superiority of the offensive to the defensive in land warfare and advocated deep attacks to disrupt and defeat an enemy. (If the enemy is attacking, you find his weak spot and counterattack to throw him off.) This contrasted with the prior Active Defense doctrine (1978 edition of the the same field manual) which had emphasized the ways a defender can inflict crippling losses on an attacker. (If the enemy is attacking, you find the strongest spots to defend and massacre him there.) In addition, many NATO countries were building up conventional capabilities at this time and fielding new generations of armored fighting vehicles, tactical aircraft, etc. that seemed to have greater offensive possibilities.

Thus, questions emerged in media and political circles, as well as to some extent among the public, as to whether NATO was now on an offensive posture. NATO's strategy for defense of West Germany at that time was Forward Defense: try to defend as much as possible. Would "forward" include somewhere in East Germany?

So a few months after the FOFA announcement and in response to those questions, the Supreme Allied Commander Europe (the commander of NATO forces charged with defending Europe, always an American officer) General Bernard Rogers published an article entitled "Follow On Forces Attack: Myths and Realities" (found among other places in the December 1984 issue of NATO Review) in which he stated that FOFA did not involve land forces; AirLand Battle was an American not NATO doctrine, and NATO would not mount a ground invasion of enemy territory even in wartime.

This apparently reflected a ban on any NATO military planning that could be construed as offensive in orientation. Lieutenant General William Odom, who was G-2 (intelligence officer) of the whole U.S. Army until 1985, when he was appointed head of the National Security Agency, confirms such a ban in his 1998 book The Collapse of the Soviet Military, though he also states that he does not think the Soviets believed it.

In any case, I am not aware of any such plans having been found. Likewise the scenarios for NATO large-scale wargames nearly always followed a script that went like this: the Warsaw Pact invades NATO territory, NATO defends, and there is escalation to nuclear warfare while the frontline is still in NATO territory. It is possible national armies or units had such plans, but the documentation has not been made public.

Did the Warsaw Pact have military plans to invade NATO territory. Yes. We have found such plans, in particular those in the archives of the former Warsaw Pact countries. (I am not sure why you say otherwise.) The most well-publicized one is called "Seven Days to the River Rhine" which was revealed by the post-communist Polish government. The Warsaw Pact also conducted wargames whose script began with a weak NATO invasion of Warsaw Pact territory that was repelled, resulting in a larger-scale counteroffensive into NATO territory, whose prosecution formed the bulk of the exercise.

This orientation reflects the fact that Soviet military doctrine had always preferred the offensive mode of warfare. They had been on both the offensive and defensive during WWII and found the offensive much more agreeable. So even if NATO were the aggressor, the Pact would attempt to go over to the offensive as soon as possible and push into NATO territory, and had no qualms about saying so.