Maven artifacts are not immutable. Some maven repositories may prevent overwriting an already published version, but this is not guaranteed. I've personally seen this cause problems where a misconfigured CI job overwrote already published versions.

npm used to allow you to unpublish (and may be overwrite?) published artifacts, but they removed that feature after a few notable events.

Edit: I was not quite correct. It looks like you can still unpublish, but with specific criteria. However, you cannot ever publish a different package using the same version as an already published package.

https://docs.npmjs.com/cli/v8/commands/npm-publish?v=true

https://docs.npmjs.com/policies/unpublish