They have been testing it for over 4 years in Shanghai already. They replaced their regular fastpass with the standby pass in 2020.
Disneyland Paris is the second park they are doing this.
Disneyland Paris got a few years ago only the package deal option Shanghai had, last October they started with the standby pass. The pay-per-ride FP was the last step.
Disney's actions on how they role this out and where makes perfect sense, Shanghai was new, their audience didn't know anything else. Taking it to Paris next makes even more sense as in culture Europe is closer to the US than Asia. They will use these two case studies to perfect it before bringing to their biggest / most important audience.
It wouldn't make sense to test it in Hong Kong as their numbers are much smaller in comparison to Shanghai. I love Hong Kong and it's my favourite Asian park, but if it is about data collection Shanghai is a better option. Disney doesn't own Tokyo. WDW is their big tourist destination, you have to be careful before you introduce anything here. That leaves DL, where they just tested Maxpass, also for the last four years. This was a very soft launch to introduce the concept of paying for premium access to the US public. Very low price, some nice benefit with the Photopass. Like introducing a child to spicy food by slowly adding some more spice in each meal.
Before they go bigger and charge more in the US, they need more and more data. Shanghai and Paris are going to provide that.