That promise was:
"If you're using iOS or iPadOS 14.5 or later, you might now see the option to choose between two software update versions. This option allows you to choose between updating to the latest version of iOS or iPadOS 15 as soon as it's released, or continuing on iOS or iPadOS 14 while still getting important security updates for a period of time."
Although the promise specifically states "for a period of time" many users assumed it would be forever or at least indefinitely. It isn't and Apple just pulled the plug on iOS 14 support. There won't be any more.
In some ways that's disappointing but it's not really a surprise. It is disappointing because most companies continue offering support to older products for a full year at least and in Apple's case it hasn't been that long.
On the other hand, it is not surprising because it's clear that Apple really wants people to upgrade to iOS 15. If it gets a little too heavy-handed to make that happen the company has now demonstrated a willingness to do so.
Unfortunately, based on Apple's own statistics fully 30 percent of their user base is running iOS 14. That means by pulling the plug they're essentially leaving about a third of their user base to fend for themselves in terms of security updates they're no longer getting.
Yes this will provide a powerful incentive for them to upgrade but given Apple's reputation, that seems a bit out of character.
At the root the issue is that Apple wasn't transparent with the timeframe. "For a period of time" could mean almost anything. Apple clearly meant for it to mean "for a short period of time" but many of their users didn't interpret it that way. As a result, there's bound to be some bad blood over this.
In any case if you're currently running iOS 14 upgrade as soon as possible because no further security updates are coming.