Did you try this?

parameters:
    env(TRUSTED_HOSTS): "10.0.0.1,10.0.0.2"
framework:
   trusted_hosts: '%env(TRUSTED_HOSTS)%'

The container build doesn't complain with your example anymore, but the array is not resolved:

var_dump($container->getParameter('kernel.trusted_hosts'));
// array(1) { [0]=> string(17) "10.0.0.1,10.0.0.2" }

And the same for the CLI:

bin/console debug:container --parameter=kernel.trusted_hosts

 ---------------------- --------------------------
  Parameter              Value
 ---------------------- --------------------------
  kernel.trusted_hosts   ["%env(TRUSTED_HOSTS)%"]
 ---------------------- --------------------------

And if I try to configure trusted_headers the same way, I still get the error:

The value "x-forwarded-for,x-forwarded-host,x-forwarded-proto,x-forwarded-port" is not allowed for path "framework.trusted_headers.0". Permissible values: "forwarded", "x-forwarded-for", "x-forwarded-host
  ", "x-forwarded-proto", "x-forwarded-port", "x-forwarded-prefix"

My example doesn't contain any array.

Okay, so is the documentation wrong and this cannot be done with the json or csv processor?

Environment placeholders are not supported for all bundle semantic configuration. It depends whether the bundle processes them or passes them as is to the ContainerBuilder. They are supported only when passed as-is (as their value is not known at compile time so any node processing the value is likely to fail or do weird stuff)

The documentation is completely wrong yes :)
It's wrong because if uses the env var in a way that doesn't work, and it's also wrong because trusted hosts is expected to be a list of regular expressions. This page is correct:
https://symfony.com/doc/current/reference/configuration/framework.html#trusted-hosts

Can you please a PR to the doc to fix this? I suppose we need to find another example.

I went through our whole project, but I can't find anything useful as an example. I'd have to make one up 😄

Maybe some arbitrary list of language codes:

parameters:
  env(ALLOWED_LANGUAGES): '["en","de","es"]'
  app_allowed_languages: '%env(json:ALLOWED_LANGUAGES)%'

Somewhat related to #50323

@nicolas-grekas I've opened a PR with the arbitrary languages example: symfony/symfony-docs#18332

Thanks!