Overall status update:
ISRG Root X2 is included in:
- NEW Apple: ISRG Root X2 is now included in Apple trust stores when running macOS 12.3 or higher (source)
- Google (Chrome): An update to the Proposed Root List somewhere in early 2022 started to include ISRG Root X2 as part of roots included at launch. AFAIK, the Chrome Root Program/Store is still not live, but Google has plans to launch it later this year.
- Mozilla (Firefox/NSS): ISRG Root X2 is included as of Firefox 97 (see news above in this thread)
- Microsoft: Windows distributes ISRG Root X2 since 2021 (see above). However due to path building specifics & lazy loading issues it may not show up in your system.
ISRG Root X2 is not yet included in:
- Oracle (Java): Latest OpenJDK sources do not include ISRG Root X2 yet .
- Android: AFAIK, there is no formal Android root program. However, Google does maintain a system trust store for its operating system (it's unknown to me from which program this is sourced from). Sources available from AOSP (corresponding to Android 12) do not appear to include ISRG Root X2.
Note that even if a root is included somewhere it takes years before these updates have reached (almost) all users.