This is a contentious issue, as you say Erik did a very good (the best!!) wake-up library and although this concept has been attempted in later Mbed libraries right up to v6, I’m afraid it is a no go now.
There’s way to much emphasis on thread based architecture in the current mbed OS.
Yes you can do quite a lot of low power projects but there are limitations.
Unfortunately for Mbed there are better solutions out there , the rapid development side of mbed has gone a long time ago.
There’s been way to much depreciation and to many ‘RAPID’ library changes that makes mbed the underdog now.
Dam shame, I love the online compiler and Studio (off line ).
But I need to get some work done, can’t do that on mbed unless they get new staff and a new direction.
Give Arduino a try, you will absolutely hate the IDE, but I’ve been working with some 16MB ESP32 device’s and it blows mbed out the water. Although you do have to approach low power from a different angle.