UXP is still work in progress, and unfortunately the documentation, such as it is, isn't being updated as UXP develops. The first link you gave, to the OMV, is for ID 18.0. It's mentioned there that as of InDesign 18.0 the ESTK no longer reliably connects to InDesign. In my experience that's not the case, in ID 2025 it still connects fine (on Windows) and its object-model viewer works as it always did (i.e. warts an all). On MacOS you have the ESTK extension for VS Code, which since recently sports a well-working OMV. The second link, the UXP overview, was last updated in 2023, like other pages I saw last week. All out of date.
It's true that ExtendScript is based on an old JavaScript version, but that doesn't mean that you can't use it. In fact, it is very much usable.
As for deprecation, how long ExtendScript and CEP will be supported is not clear, probably even Adobe don't know how long they'll be with us. UXP (both scripting and plug-ins) is by no means finished and won't be for a while, so ES and CEP can't be cut off. Another reason for continued support is the zillions of ExtendScript scripts out in the world which Adobe wouldn't want to disable.
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