I’ve come across an interesting observation.
I installed the iOS 27 Beta 1 developer build on my iPhone 15 Pro Max the day after the keynote, so exactly one week ago. Despite this, the device remained in the “Still Indexing” state and I was still on the Siri waitlist right up until this morning.
Today I replaced the phone with a new iPhone 15 Pro Max after damaging the back of the original device. Once the replacement was set up and iOS 27 was reinstalled, I noticed that the Siri icon in Settings initially displayed the new Apple Intelligence/Siri branding, even though launching Siri still presented the older Siri experience.
After approximately an hour, the phone reverted to showing “Still Indexing”, the Siri icon automatically switched back to the previous version, and I was placed back on the waitlist.
It appears that the device briefly recognised eligibility for the new Siri experience before reverting to the standard provisioning process.