Remove the DEP0018 deprecation

[mike-marcacci]

Is your feature request related to a problem? Please describe.
Deprecation DEP0018, which is associated with the poorly named UnhandledPromiseRejectionWarning, is misleading and fundamentally incorrect. Asynchronously handling promises cannot be deprecated, as it is a basic tenant of the promise spec.

When a promise that is handled asynchronously is rejected before the handler is attached, the following is currently logged:

(node:45) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:45) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Describe the solution you'd like
While preserving the warning may be useful, the deprecation (which will never happen) should be removed.

When a promise that is handled asynchronously is rejected before the handler is attached, only the following should be logged:

(node:45) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)

Describe alternatives you've considered
Finalizing this proposed deprecation is tracked by #20392. This proposal is deeply flawed and would have far-reaching consequences if implemented. In #31148 I proposed a strategy to address the same pain points in a correct, more generic way. Regardless of whether my approach is taken or not, async handling of promises is a core feature that simply cannot be deprecated, and we should remove the erroneous deprecation accordingly.

[mehmetkoc182]

Looks like there is some push back on this mostly due to performance concerns. Would it make everyone happy if you implemented this behind a flag?

[jasnell]

At this point, the overwhelming and stable consensus within Node.js core has been to maintain the deprecation. However, it is possible to use the --unhandled-rejections=none command-line argument (and within the NODE_OPTIONS environment variable to achieve the behavior you're looking for..

$ node --unhandled-rejections=none

$ NODE_OPTIONS=--unhandled-rejections=none node

[jasnell]

Closing as there is no further action to take on this specific issue. The strategy around unhandled rejections is being refined in other PRs and issues