I got a manuscript published in November 2018 (Elsevier) and realized after reading it again today that an error has been introduced, after proofreading/publishing, in one of the equations I wrote; they have added a "Â" right after "f(x)=" which is nonsense. Is it conceivable to ask for correction now?
Also, two of the texts describing patterns in the legend of a graph were interchanged; this one is my mistake (prior to publishing).
Typically, it is difficult/impossible to correct this kind of mistakes after formal publication. The 'traditional' reason is that there are now printed copies of your article in libraries around the world, and the online copy should coincide with them. Today there are online-only journals, for which this reason is less compelling, so you might want to ask, but I wouldn't be surprised if the journal answered that it's their policy not to change anything.
For something more major, you would issue an erratum / corrigendum, but for typos and small mistakes there isn't a good solution.
