The only reason Stones wasn't a regular starter was injuries. I don't see any drop off in his ability.
Stones was fit in April and May 2024 when we were fighting to win the 4 in a row, but Pep started him only once, Palace away (early April). Pep started him in the FA Cup semi vs Chelsea, and in the final vs United, but he wasn't great.
Stones had 6 PL starts in 24/25, we lost half of those games, and experienced two of the biggest defeats under Pep: 0:4 vs Spurs and 5:1 at Arsenal. Nothing to write home about. Generally, last season our league results were better without Stones than with him. But since the sample is too small, one can't draw firm conclusions about his form. Apparently, it wasn't great.
This season, Stones started 3 league games, we lost 2 of them. One can't seriously claim he was great vs Spurs or Brighton. From the 4 pl/cl games he started we won one: at Wolves.
So, he hasn't been great since April 2024, if not earlier. You can't say he has been at the same level as in 22/23. What's more, our league record with him starting games has been worse than without him in the first XI (since 23/24).
I hope Stones regains his 22/23 form and helps the team compete for the big trophies. He should stay fit for at least 2 months. Otherwise, it would be a mistake to make him our 1st choice CB.
Edit: our record in the PL with and without Stones starting (since April 2024, last 18 months or so):
with Stones: 1,5 ppg
without Stones: 2,18 ppg