The evidence is very clear that it's beneficial if you have a dietary deficiency (which you almost certainly do). If not for the proven physical benefits, the cognitive benefits alone are likely worth it. Where the evidence is ambiguous is how beneficial it is for someone who meets minimum daily dietary intake to then supplement it.

I have no doubt about the effects of a creatine supplement on the muscles, as there the action mechanism is well understood, but I am more skeptical about the claimed cognitive effects, because the mechanism for such effects is not known yet.

Moreover, for substances like creatine, which can be produced in limited quantities by the human body, like also for DHA, EPA, choline or taurine, there may be large differences between individuals. Some people might not need supplements for one or more of these substances, at least when they are young, while for others supplements may be indispensable, at least when they become old.

In general, it is much more prudent to take a supplement, even when its necessity is not certain, than to not take it, because in the first case if your choice was wrong it just does not have any useful effect, while in the second case if your choice was wrong it could cause serious health problems.

I have already thought that I should try sometime in the future a creatine supplement, to see if I notice any change, but for now I cannot not see any cognitive differences between present and how I was before switching to an 100% vegan diet.

first of all, creatine is a non-essential amino acid; which our bodies can synthesis accordingly our needs

Yes it can, in very small amounts. You'd have to eat a ton of the synthesis precursors as a vegan to obtain anywhere near the optimal amount or to approach what a meat/fish-eater gets in their regular diet.

The literature is abundantly clear on this.