Can only speak for myself, and no, the letters do not go in my signature. The degrees got me in the door, and help me do my job. At the time, the second degree almost disqualified me from getting the entry-level job, they thought I would be overqualified. nowadays, it's rare that a person in my field(s) of expertise doesn't have one advanced degree, again, the job expectations these days virtually require training above the 4 year degree, but some can still come in the door without it and succeed up to a point with mentoring support and ojt and continued reading of new professional information pertinent to the job.
But one degree higher than my second degree, people with those letters do often put them behind their name in their signatures. those individuals also work at a higher level or in different branch of my agency, one where those particular letters actually can be more meaningful behind one's name. It's still a judgement call when the lower ranks see such a signature, how they react or what they privately think about it.
At my level and one level down, people will think you're overly proud of yourself if you put letters behind your name in your sig. It's what you do with that education in the post-academia job that matters to colleagues, not the degree itself. I tell newcomers with that 3d-level degree that it will not benefit them in their relationships with coworkers to stick it in their faces if they are going to work at my level or a couple rungs lower. It's ok if their colleagues know, or heard through the grapevine, but they don't need to hear about it or see reminders on a routine basis. When in doubt, leave it out. a good rule for many communications situations.