Sony has established some clear patterns when it comes to the PlayStation Plus subscription service, revealing the new free games on the last Wednesday of every month and adding the next batch on the first Tuesday of every month. Since the launch of the PlayStation 5, PS Plus has usually consisted of two free PS4 games and one free PS5 game in any given month, with some exceptions. However, some PlayStation Plus subscribers are seeing an extra free game listed in the PlayStation Store, though it's not possible for them to claim it.
Some PlayStation Plus subscribers have reported seeing Tony Hawk's Pro Skater 1 + 2 listed as one of the free PS Plus games in the PlayStation Store. However, when they try to claim Tony Hawk's Pro Skater 1 + 2 for free, they are asked to pay for it like they normally would. This is a bug that pops up every so often in the PlayStation Store and while it's usually not indicative of future free games, it does have fans speculating.
There was a rumor a couple months back about Tony Hawk's Pro Skater 1 + 2 being a free PS Plus game, though it turned out there was no truth to those claims. However, Sony and Activision have a solid working relationship, with Call of Duty: Black Ops 3 and Crash Bandicoot N. Sane Trilogy available on the PS5 as part of the PS Plus Collection, and other Activision games made available for free on PS Plus over the years.
In fact, PlayStation Plus has enjoyed multiple Call of Duty games as freebies, which is significant as Call of Duty is consistently the top-selling game every year. Most recently, Call of Duty: Black Ops 4 was a free PS Plus game, and so one when considers the history of Activision titles being made available for free through PS Plus, it's easy to believe that Tony Hawk's Pro Skater 1 + 2 could be a possibility at some point down the line.
Regardless of whether or not Tony Hawk's Pro Skater 1 + 2 is ever made free through PS Plus, though, this bug is not evidence in favor of it. It's just a random bug that happens with PlayStation Store listings sometimes, and it will likely happen again.
In the meantime, the actual free PS Plus games for September 2021 are currently available for fans to claim for their collection. They are Overcooked: All You Can Eat on PS5, and Hitman 2 and Predator: Hunting Grounds on PS4.