Nobody is undroppable if their form is bad enough.
Most players are likely to get a break if they need it for prevention of fatigue related injury problems.
The player whose skills are hardest to replace in my view is Rodri.
Stones
Haaland
Ederson
KDB
Bernardo
In my view would play every important high level fixture no matter what tactical option Pep chooses if fully fit.
Foden
Walker
Akanji and possibly Dias are next in his pecking order
There is no surprise or disappointment if in a vital match with a fully fit squad any of
Doku
Grealish
Ake
Gvardiol
Alvarez play but equally it is not surprising to see them on the bench.
Lewis, Ortega & Kovacic are very capable players if others are rested or injured but probably don't make a starting 11 with everyone fit and no one needing to rest in the most important games.
Nunes is currently unproven
Bobb and Gomez are currently developmental players.
Phillips seems to be purely for minor matches and as a substitute to rest others when a match is won.
Is the way I currently see the squad.
With alterations in form and fitness it could change.