I think that the keyboard command CTRL + Shift+ Spacebar results in a non-breaking space in text added via the Modern Text web part , for example if you want dates or phone numbers to stay together and not wrap over a line.

However, most of my authors do not know keyboard commands, and when text is copied an pasted from Word, any non-breaking spaces seem to get stripped out of the text. I'm okay with the style stripping, but it would be good if the text web part included a WYSWYG feature that allowed users to add in non-breaking spaces - and also symbols like en dash, copyright, trademark and the like.