The evidence is shaky at best; the consensus is that there certainly were plans for covert NATO-coordinated stay-behind forces to be activated in the event of a Soviet invasion of Western Europe, but the program seems to have petered out fairly quickly and never participated in any active false-flag operations. Plus, the NATO-led operation seems to have been mostly focused in and around the United Kingdom, with only tangential involvement in Italy and other european countries, who instead developed their own covert stay-behind operations under the "Operation Gladio" umbrella. Italian Prime Minister Giulio Andreotti, in his famous 1990 testimony, only ever referred to the Italian "Gladio" forces as a wholly domestic project which had ceased operations by the time the "Years of Lead" began, although the heavily redacted documents released that pretty much halved the project's manpower did little to subdue suspicion with regards to the project. In addition, there is some doubt as to the proper disposal of the weapons stores kept for Gladio, meaning that new evidence might very well prove the project was in some way indirectly responsible for arming extremest terrorism some twenty years after its cessation.

The extent of academic analysis on the Gladio operation's connection to terrorism is almost wholly attributable to the Swiss historian Daniele Ganser at the University of St. Gallen, who published "NATO's Secret Armies: Operation Gladio and Terrorism in Western Europe." The fundamental problem with that book is that it is based on the US Army Field Manual 30-31B, which was revealed to be a piece of Soviet disinformation propaganda produced in the early 1970s, just when the "Years of Lead" were starting.

If you'd like to read more, this discussion between and u/commiespaceinvader and u/lazespud2, the latter of whom's answers I have drawn from extensively to formulate the statements above, had a very in depth-discussion awhile back, however I'm very familiar with the history and politics of post-WWII Italy and am more than happy to answer any additional questions (and probably so will they!)