MicroPython tries to maintain backward compatibility. (old) books and youtube videos do not sound like a reliable source.
There is always the Online documentation at https://docs.micropython.org/en/latest/.
And you can always ask here.

Can you provide an example of some code that isn't backwards compatible? If a breaking change hasn't been documented clearly, we may be able to make improvements...

You can also download older, compatible firmware from https://micropython.org/download/RPI_PICO/

The oldest version available is v1.14 (released 2 February 2021).

I maintain a number of MicroPython libraries, some containing code written 9 or 10 years ago. Some of these are widely used, so if a firmware regression arises I soon get to know about it. Such issues in straight Python code are extremely rare.

The point here is that MP aims for CPython compatibility: if you code according to CPython rules you will be unlikely to hit a regression (and the maintainers will fix it). So one cause of fragility is writing code that would fail under CPython.

Another possible cause is early code (in the history of the RP2040) that uses advanced features of the chip such as the PIO. It is possible that the support for such features may have changed since it was first published, or that early code may rely on bugs in the PIO API which have since been fixed.

Another gotcha may be trying to run stm32 code using the pybd module, rather than the machine module on the rp2 Pico