Nope, since ps+ was introduced it’s always been the case that the monthly games are yours to keep forever.
The games on the premium tiers are like gamepass, you have to keep paying to access them and games come and go from the service. But the monthly games are yours to keep
That’s definitely not true. I had PS+ back in 2014/2015 and all the monthly games I’d added to my library were locked when I canceled it. Per Playstation:
Once your PlayStation Plus subscription ends, content you previously downloaded at no cost as part of the subscription (such as monthly games) will no longer be available.
I regained access to them once I renewed my subscription later, but that’s not the same thing as “games you get access to forever, even if you stop paying.”