To address your assertions :
a clerk said that choosing iphone 7 would be unwise, as it's old and
in few years it will stop receiving updates
That is a correct assertion from a software perspective. Apple dropped support for iPhone 6 (2014) in 2019 when launching iOS 13. iPhone 6s (2015) seems to still be supported in 2021 for iOS 15 (but that's a surprise).
There is no guarantee that the iPhone 7 (2016) will support iOS 16, even less iOS 17. You can't expect to buy a 5 year old smartphone and keep software support for 5 more years.
Note that when I talk about software support, I'm talking about the latest version with up-to-date features. There is an history of Apple releasing security fixes for older iOS versions, but again, there's no guarantee on this.
device's battery lifetime degradation and stuff should start from the moment it was assembled
That is correct, battery degradation starts at the time it is manufactured. But note that the iPhone 7 is no longer manufactured, it has been discontinued in 2019. Even if you can find a brand new iPhone 7 now, it must have been sitting on the shelf for at least 2 years so the battery is definitely not in a brand new state.
Those two aspects are very good reasons not to buy an iPhone 7 in 2021 if looking for a brand new phone and would certainly explain the clerk position. If however you can get a very good bargain and don't mind the software support, you can definitely buy one now, they are still fully functional smartphones.
And to address your final question :
Are there credible technical basis for reports of “Apple deliberately
slows down older devices when new software is released”?
There is two aspects of this, the first is software, the second is hardware.
For the software aspect, all update includes a new set of features with increasing computational complexity. Those are designed to take advantage of new hardware but may not operate as smoothly on older hardware. That is true for iOS but also for any other operating system (Android, Windows, etc.).
The longer you keep software support for older hardware, the higher you risk slowing down those older devices at each software update. Apple has an history of keeping support for older devices (often longer than other mobile OS) which can have a consequence of slowing down older devices, wether it is deliberate or not is a matter of opinion.
For the hardware aspect, when a battery gets older, it not only loses its capacity and drain more rapidly, it also loses its ability to sustain high current. If a battery is worn out enough, it may not be able to provide the power required by the smartphone on heavy tasks. This phenomenon may lead to random shutdown if no other countermeasure is implemented.
The measure implemented by apple was to limit the CPU power on older devices to avoid these random shutdown. This have the consequence to slow down the phone. This raised some backlashes and can now be turned off (if performance is preferable to system stability).