webRequestBlocking is not deleted in MV3. The synchronous blocking engine still exists in Chromium. If an extension is installed via local policy or the Windows Registry (ExtensionInstallForcelist), full programmatic webRequestBlocking executes in Manifest V3 just as it did in MV2.

It is in practice.

Not all users are able to do that (local policy install), as not all users are (or have to be) that tech savvy. That's an exclusionary barrier towards inclusion and accesibility of security and sovereignity features.