Is JXA still supported?
Is JXA still supported by Apple? Does it have a future in macOS?

JXA is still supported.  In macOS Catalina, JavaScript is still an option for automation.  It's also present in Big Sur.  More importantly, there's been no indication or statement regarding it being deprecated † or removed as a feature from Apple.
Does it have a future in macOS?
In all practicality, only Apple can answer this. However, IMHO, it's likely to continue for a long time.  JavaScript is a very popular and widespread language meaning there's lots of resources available.  According to the Stack Overflow Developer Survey for 2020, it was the top in programming, scripting and markdown languages (eighth year in a row).  It would be short sighted to kill it from AppleScript.

Regarding the documentation, (again, IMHO), there's no need to update it.  What was linked there was an intro and nothing has changed since it's introduction.  Anything related to JavaScript is already well taken care of with the countless resources available.

† There's a distinction between being deprecated and dead.  Deprecated being &quot;no further development but still supported&quot; while &quot;dead&quot; means it's gone.