In the Plugins Documentation, there's a warning section indicating that:

Plugins are not supported by the ADK web interface. If your ADK workflow uses Plugins, you must run your workflow without the web interface.

In another section, within the same page, there's also another reference:

Plugins are not supported by the ADK web interface. If your ADK workflow uses Plugins, you must run your workflow without the web interface.

which I don't think it's accurate.
In fact, it is possible to bind plugins via the --extra_plugins argument when running adk web:

  --extra_plugins TEXT            Optional. Comma-separated list of extra plugin classes or instances to enable (e.g., my.module.MyPluginClass or my.module.my_plugin_instance).

I find this a bit misleading, as it gives the impression that it's impossible to run plugins via adk web.