I don’t know either of the HA’s. But, I’m guessing the Oticon Xceed were MFI, which gave you directly accessed the Accessibility shortcut for HA’s.
So, I’m (still) guessing that the Phonak Shy M70’s are not MFI and therefore no Accessibility shortcut. At the moment, there is no way of accessing anything that is not currently listed in the ‘Control Centre’ (which are accessible while the phone is locked), including widgets.
There is an alternative if you have Siri activated. You can ask her to start the myPhonak App, by name. This does work (just tested). This also implies that you are running face detection, and your phone is already unlocked on the way in.