Do people still have to write thesis after finishing their master/phd? I know that the thesis is needed when doing their master.
But do people still need to write a thesis when doing research as a past-time?

What if they just want to publish an article but not thesis?
No, people don't have to write theses after the Masters/PhD. The thesis is a formal requirement for those degrees, but after that, it's just not necessary anymore.

Actual research papers, though ... those are effectively "compulsory".