I'm opening this because I've seen fragments of this request dotted around other issues, but I couldn't find a single issue that tied them together.
The current setup of the docs site is confusing and causing frustration. It looks like the main site now contains documentation for v5, which is still only in beta right now. The only indication that the docs are for the new beta version are small notes in the left column.

I just got hit by the exact same problem as https://github.com/webpack/webpack/issues/10716 — I went looking for a config option, tried it out, then hit a compilation failure because it turned out to be a v5-only option.
Rather than just complaining, I have some suggestions:
v5.0.0 is actually released, with v5 docs as the alternative site (so the inverse of what's currently set up). Running npm install webpack still installs version 4 at the moment, so that's what a huge majority of people are going to want documentation for. It's really confusing to have the installation and docs be out of sync like this.How about adding a version badge indicating which version of webpack has introduced that option?
For example:

Hi @gilmoreorless
Current live website is for webpack 5 which is the recommended version for the projects due to numerous improvements. The shield that has a beta on it, indeed does indicate the latest version of webpack that docs cover
@EugeneHlushko Thanks for the response. But I feel you've just repeated my opening problem statement. Maybe I didn't list the problem clearly enough:

Closing this since v5 has been released.
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@EugeneHlushko Thanks for the response. But I feel you've just repeated my opening problem statement. Maybe I didn't list the problem clearly enough: