It's a fundamental and unavoidable part of CRT that they have to scan the picture, line by line. 100hz processing increases the speed at which this is done and reduces flicker.
There's nothing fundamental about Plasmas that forces them to scan like CRTs do. So, it is theoretically possible to make a wholly flicker free Plasma.
However, I can say, as one who is able to see flickering up to about 80hz, that Plasmas vary in this; some have (to me) no visible flicker, whereas others do so quite noticably. The worst culprits were Panasonic although I suspect this has changed with later models.
100hz is only essential (to avoid flicker) on CRTs. Depending on how the pixels are energised, a 50hz Plasma does not need to flicker (although, as I say, some do).