This.
Y7s enter secondary school as children and leave (assuming y13) as adults (although y11 isn't far off). It's a phase of life building those skills to be independent, functioning adults and they need the tools to make that transition in the world they live in.
Quibbling over smart phones/ dumb phones is usually semantics when the negatives of smart phone use can often be experienced through other forms of tablet/ pc/ gaming system.
The problem is largely social skills/ awareness.
I went for the delay until y6 approach. Christmas y6 gave chance for a couple of terms of learning curve in the closed environment of primary school where the stakes are a bit lower than secondary school and problems easier to identify, flag up, manage and learn. DS was already walking a short distance to/ from school but didn't greatly matter about phone access so there was no rush.
Secondary is different. He needs access to a phone with an app to use the bus. There are no payphones on the 3 mile journey home, or safe places to pop in to make a phone call. We had one day of a storm with trees being down, bus services heavily delayed and diverted. Being able to access the bus website and google maps was a godsend. Timetables and maps have disappeared from public spaces along with the payphones.
We use parental controls and manage app usage. This is not foolproof but I'd rather build up the practical and social skills gradually rather than ignoring until late teens and hoping for the best at a time when I have less influence.