It really depends on the type of plastic. For example your drugstore 70% alcohol will cause severe crazing and cracking of clear acrylic, which many motorcycle helmet owners have discovered the hard way. Drugstore 3% hydrogen peroxide and household bleach each can damage acetal and Nylon 6/6 as on a Glock.
I have to laugh at those shavers with the special disinfecting cycle requiring special proprietary fluids--I am not sharing a shaver with anyone else and am not afraid of my own germs. Likewise, if it's your own keyboard then why does it need disinfecting? If however you work in shifts and other people in the office may be using your keyboard, then clean away--if the letters come off then your employer should replace it.
BTW alcohol will soften many paints and silicones, so rubbing when it is soft can indeed remove painted-on letters over time. One solution is to buy a better keyboard with double-shot injected keys where the lettering is a solid plastic color all the way through the keycap (this is also how keyboards with backlit lettering are made--the font color is clear plastic instead)