I want to know if I'll encounter any problems if I'll open old .psd file that doesn't have "Maximize Compatibility" in newer version of Photshop.
Is this function only for older Photoshop versions so they can read the files or does it affect new versions in some way as well?
Photoshop is backwards compatible - in other words, newer versions of Photoshop can open older PSDs, but not always the other way round, since newer versions of Photoshop often have additional features not available in older versions.
That's why there is a "maximise compatibility" option - so you can open PSD's generated by newer versions of Photoshop in older versions of the software.
Therefore, there should be no problems opening older PSDs in newer versions of the software. I've certainly never come across any problems while doing so.
