is it true that the newer mac OS' are difficult to use on older devices because apple wants people to buy new hardware?
You actually ask two questions which may or may not be related
is it true that the newer mac OS' are difficult to use on older devices
Yes, probably. But the reasons are variable.
For example macOS High Sierra, macOS Mojave and macOS Catalina all use the Apple File System (APFS) and this can be problematical for Macs with spinning platter hard drives. Even worse if, by age or design, these are at 5400 rpm rather than 7200 rpm.
The same goes for RAM. Those Macs that shipped with 2GB or even 4GB will struggle with the later Operating Systems - generally 8GB is advised as a minimum. In some Macs, upgrading the RAM is fairly easy, in others, difficult to near impossible.
CPU and GPU specs gain some importance too.
The age of the components, especially Internal Hard Drives, is a factor. So, lots of reasons why a particular Mac might have difficulties, but these vary according to the parameters which I've mentioned above.
because apple wants people to buy new hardware?
Every company making any physical product depends on people continuing to purchase that product; but I do not believe that Apple deliberately sets out to force people to discard their old for the new.
There are many on our Forums happily running Snow Leopard (10.6.8) for example because does what they need and it "ain't broke".
So, I return to the point that your two questions are only loosely related.
Ian