An unstable interface means the driver source needs to be updated frequently, you can't just dump a .ko file online and expect it to work for however long the hardware lasts.
Easiest way to approach it is to attempt to upstream drivers, and potentially take advantage of free labor and maintenance in virtual perpetuity, which is good for all Linux users. If vendors don't want to spend the effort upstreaming drivers, but they need to support Linux, by necessity the drivers must be open source so they can be compiled against users' changing kernels. That's at least a step in the right direction, and should anyone want to make the effort, they're free to upstream drivers themselves.
So don't repeat these legends from 20 years ago. However, this may not have been true even 20 years ago.
Yes, bad driver implementations that shit all over the kernel tree just to get hardware "working" should not be upstreamed. 99% of the time in these cases, Chinese equipment manufacturers can't be bothered to write acceptable code and it's a good thing it isn't mainlined and made someone else's problem.