OK, here is my solution, even though it is an ugly one. For reasons not explained anywhere MG 3.3+ no longer includes namespaces
Mariasek.Xna.Framework.GamerServices. This means that support for the soft on screen keyboard input is not part of the nuget package. There are some old articles on the web stating that these have moved to a separate nuget package called Mariasek.Framework.Net.dll - this is not true any longer, as of end of 2017 there is no other package to my knowledge that contains this functionality available on nuget.org
Should you want these features you have to download MG from GitHub and build it yourself and then include references to
MonoGame.Framework.Net.dll in your project - the latter dll contains the
.Net namespaces. You cannot use the dlls from the nuget BTW as at least for iOS there are some missing references.
Once you have done that you may call
Guide.BeginShowKeyboardInput(...) just note that the function does not support passing of the state object contrary to what the XNA documentation claims - so should you want to pass a state object to EndShowKeyboardInput() you have to use other means to do so (like a private field or property, be careful about thread safety).
Hope this helps!