I saw that vegans often use the word mylk instead of milk, like in this answer.
Why do vegans call milk mylk?
This non-standard spelling is used, in my opinion, because of the discomfort some vegans feel with the idea of dairy milk as food and the desire to distinguish vegan "milk"s from dairy milk, but also, the desire to see vegan m[iy]lk as a food in itself, not as a substitute for dairy milk.
Many vegans are interested in effecting social/cultural change, and language is always a vehicle for such change. When I specify "vegan milk", I am tacitly admitting that the "milk" I use is a substitute or deviant variant of "the real thing" or the "normal" milk.
If I want to normalise my own practice, I might seek to avoid this kind of specification, and instead try to replace the "normal" of dairy milk, with a new "normal", vegan milk. When I say "milk", I want it to mean "vegan milk". However, since the hearer is unlikely to understand it that way, I am forced to specify, and mark my practice as different from the norm.
In writing, on the other hand, I can attempt to distinguish the milk that I use and accept as food from dairy milk which I consider unacceptable, without marking my practice as marginal, by deliberately mis-spelling the word. I signify that I want to replace one practice with another.
Thus, the spelling "mylk" for milk should be considered a quiet act of protest against the use of dairy milk and the centrality of dairy milk as a food in some cultures.
It also saves time/effort/bytes etc, especially when writing a recipe for vegan milkshake (mylkshake, of course)
This practice mirrors plant-based alternative products often sold under such (similar but recognizably different) names - for marketing or often legal reasons. In various localities, terms for certain foodstuffs are well defined in food codes and laws - even calling soy milk soy milk can cause trouble where "milk" is narrowly defined; any product bearing that name without matching the food code definition will be considered non-compliant or even as an attempt at counterfeiting or deception.
And as mentioned above already: A vegan who recommends "drink some milk", even if implying a plant based product, does not want to be understood as recommending you drink animal milk.