PCB with no analog and ac current and capacitor question
Given these elements: Is it still needed to put a decoupling capacitor to each ground on this DC circuit? Isn't it useful to filter high frequencies (AC) only? Do we really care about the noise a ground may have?


The way I would look at this is: "The chip manufacturers recommend them. What additional information do I possess about their chips that they don't know about?" 

The answer is "none" so I do what the manufacturer recommends in the datasheets.


  Would an ethernet PHY really need a decoupling capacitor in this circuit? Do they generate so much spikes by themselves?


If it's in the datasheet then you need it.


  I keep seeing decoupling capacitors in the schematics I read (Raspberry Pi, Arduino, and others). But their constraints are different, I guess.


It's best practice. Adopt it!