Is it alright not to include my previous supervisors as co-authors, when publishing PhD work, now that I graduated?
Authorship standards vary a lot between disciplines, but generally the hierarchical relationship between you and the other authors should be irrelevant. In other words including them before only because they were your supervisors was wrong* so is excluding them now only because you feel they have no leverage anymore.

You mention the reputation of the journals, this is not a relevant factor. It's true that some more reputable journals describe authorship criteria explicitly, something that lower-tier journals rarely do, but the expectations is always that the listed authors contributed to the presented work according to the customs of the field. 

It's impossible for anyone not closely related to your personal situation to tell you if they deserve authorship or not. What can be said is that quite often do graduate students underestimate the scientific contribution of their supervisors. 

Your only option is to discuss the matter with them. Do they actively ask to be co-authors? Have you told them that you think their contribution is not significant enough?



*I know that this still happens, in some fields too often. That's why I wrote should be and not is.