🐞 bug report

Affected Package

The issue is caused by package @angular/core

Is this a regression?

No

Description

I'm looking for feedback on this issue as much as reporting a potential bug.
If ngOnDestroy is called like this:
ngOnDestroy() { [some code] }
Then it works as expected. If ngOnDestroy is called like this:
ngOnDestroy = () => [some code]
Then it is not called.

Note that the Typescript linter is actually okay with either style, even though the interface uses:
ngOnDestroy(): void

If you're like me, you would assume that an arrow style function can be used for a lifecycle hook without actually trying it. The issue would be undetectable until you notice that the hook isn't working.

🔬 Minimal Reproduction

https://stackblitz.com/edit/whynoarrowstyle
Two components meet in the woods, one using an arrow style function, and one not using that function type. Click 'Destroy the below'. Notice that the code written in ngOnDestroy is called for the funcStyleComponent, but not the arrowStyleComponent.

🔥 Exception or Error

No error, more just looking for an explanation of the existing behavior, and whether more can be done to make it clear that using arrow style functions as lifecycle hooks will be invisibly undetectable.

🌍 Your Environment

Angular Version:

     _                      _                 ____ _     ___ 
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/
    

Angular CLI: 9.1.7
Node: 12.13.1
OS: win32 x64

Angular: 9.1.9
... animations, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router
Ivy Workspace: Yes

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.901.7
@angular-devkit/build-angular     0.901.7
@angular-devkit/build-optimizer   0.901.7
@angular-devkit/build-webpack     0.901.7
@angular-devkit/core              9.1.7
@angular-devkit/schematics        9.1.7
@angular/cli                      9.1.7
@angular/fire                     6.0.0
@ngtools/webpack                  9.1.7
@schematics/angular               9.1.7
@schematics/update                0.901.7
rxjs                              6.5.5
typescript                        3.8.3
webpack                           4.42.0

Anything else relevant?
ngOnDestroy is often used to unsubscribe, and its code can often be reduced to one line. If lifecycle hooks were expressible as arrow style functions, then we could express it in one line, like so:
ngOnDestroy = () => this.subscriptions.unsubscribe();
(Granted, you can say the same thing using classic functions as ngOnDestroy() { this.subscriptions.unsubscribe; }, but formatters love to break that into multiple lines.)