That may be the case however if a company states the minimum requirements as:
Compatibility
iPhone
Requires iOS 12.0 or later.
iPad
Requires iPadOS 12.0 or later.
Then I would expect the software to be FULLY tested based on this.
I do expect that they will indeed do as you say, but this has to come with a date and rollout to give people the chnce to either upgrade or leave.