Time-Aware Modeling, enhancements of Foreign Tables,  Ready-Only Federated Tables.

Is there any documentation on these features?

I've trained about a dozen junior to senior developers in InterSystems (and predecessors) technology.
ObjectScript is the least you should worry about. Any developer who can't grasp that isn't worth hiring.
It's the IRIS ecosystem that is completely different from 'traditional' development. Globals, Namespaces, (Remote) Databases, Mirrors, Productions, Jobs. All concepts that are not found outside the InterSystems (MUMPS) realm.
If people get their head around that, they either love it or hate it.

For what it is worth: I don't agree.
Studio is a Objectscript dedicated IDE (Editor), rather clean and unobtrusive.
It could be enhanced quite simple.
The only problem is Windows only (which is still 57.37% of all desktops)
 

Maybe you can use a generator method somewhere in your code to create the WebApp, something like:

ClassMethod CreateApplication() As %Status [ CodeMode = objectgenerator ]
{
Set $ZTrap="Error"
New $Namespace
Write !,"Creating XXX API",!
Set URL="/xxx"
Set $Namespace="%SYS"
If ##class(Security.Applications).Exists(URL) {
Return $$$OK
}
Set Props("Name")=URL
Set Props("NameSpace")="%SYS"
Set Props("Description")="XXX API endpoint"
Set Props("AutheEnabled")=64 ; unauthenticated
Set Props("DispatchClass")="API.Handler.Class"
Set Props("Type")=2 ; CSP App
Set Props("MatchRoles")=":%All"
Set sc=##class(Security.Applications).Create(URL,.Props)
Return sc
Error
Set $ZTrap=""
Return $$$ERROR($$$GeneralError,"ErrorTrap: "_$ZError)
}

Dramatic entrance ;-)
Looks awesome, definitely will have a look.

You can use Objectscript pattern matching:
Set String="a=b"
Write String?.e1P.e => 1
The case isn't important, just to highlight the 1P(unctuation)

You should use $System.Event to wake-up queue handlers, not as a queue itself.

I've reported this to WRC more then a year ago, They didn't seemed bothered.

What is the reason we cannot use Apache on Windows?
Apache is already installed, I don't like installing IIS next to it.

What do you expect of such a class (or classes)?
I might have something on the shelf as a skunk project.

Remember FHIR is a very immature 'standard'.
If you jump on the bandwagon, you must be prepared to be agile.
You talk about R5, but R6 is well on its way and, since they actually are STU4, 5, 6 and so on, there is no FHIR 1.0
Some standard bodies opt for stability like Nictiz in the Netherlands, where most of the official specifications are still based on STU3.
Opting for implementation of an immature standard ,one must be prepared for (extreme) agility, otherwise we are all ending op with in-operability because of different incompatible implementations.

1: Yes 😉
2: See other comment on Quit
3: yes 😉
You should never 'modernize' for the sake of modernizing (refactoring is something else)
ZTrap is not old-fashioned, it's a mechanism other programming languages just don't have.
It is out-of-the-way, is doesn't introduce another stack level and it doesn't need extra indentation, it's just some code that you hope will never be executed, it isn't part of the main logic.
Try-Catch can be very useful for indeed a 'Try' for something you can't be sure how it will behave, such as
{}.%FromJSON(SomeJSON)