Not to worry, friends! I just finished talking to an iOS Senior Advisor and they assured me this is certainly not true. They pointed me to this Apple support article, which clearly details this iOS 15 feature:
https://support.apple.com/guide/iphone/audio-settings-iph3e2e2cdc/ios. They assured me that if Apple removed a feature from a device, Apple would make every customer aware before they purchased the product, the Apple Support site and iPhone User Guide would be updated, and all Senior Advisors would know before any customer did, because Senior Advisors talk to Apple Engineers every hour.
If this feature isn't operating as expected, the Senior Advisor instructs everyone to restart their iPhone. If it still doesn't work, the Senior Advisor instructs everyone to force restart their iPhone. If it still doesn't work, the Senior Advisor instructs everyone to do an erase and re-install of the AccessibilityOS. To do this, you'll need to uninstall AccessibilityOS (NOT iOS) and do a fresh installation by plugging AccessabilityOS into iTunes on a Mac only. Because iTunes is no longer available, they will need to do an escalation to Engineering. Within 48 hr, Engineering will request a Data Capture. You'll need to download the Data Capture software, replicate the issue, wait 20 min, and then plug-in your iPhone. You'll then start Data Capture, with Mail Logging enabled just be be safe, and allow it to collect the logs. Once the logs are collected, you'll compress them an upload them to Apple. Once you've done that, Engineering will reply within 48 hr. Engineering will state that Apple's Official Position on the Matter is that no further troubleshooting will be provided and keep your iPhone up to date.