Both George Brotherston and WeAreModus from Laracasts are right, fbloggs.
When using Vue 3.0.0-rc.3 you need to set __VUE_PROD_DEVTOOLS__ explicitly.
This bug fixed in v3.0.7.
See Github issue resolution
See docs
Bundler Build Feature Flags
Starting with 3.0.0-rc.3, esm-bundler builds now exposes global feature flags that can be overwritten at compile time:
VUE_OPTIONS_API (enable/disable Options API support, default: true)
VUE_PROD_DEVTOOLS (enable/disable devtools support in production, default: false)
The build will work without configuring these flags, however it is strongly recommended to properly configure them in order to get proper tree-shaking in the final bundle. To configure these flags:
webpack: use DefinePlugin
Rollup: use @rollup/plugin-replace
Vite: configured by default, but can be overwritten using the define option
Note: the replacement value must be boolean literals and cannot be strings, otherwise the bundler/minifier will not be able to properly evaluate the conditions.