Issue #17121 has been reported by S_H_ (Shun Hiraoka).

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Feature #17121: Remove ENV#index
https://bugs.ruby-lang.org/issues/17121

* Author: S_H_ (Shun Hiraoka)
* Status: Open
* Priority: Normal
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`ENV#index` is deprecated since `ruby-1.9.0-0`.

So, remove it.

```bash
root@8785b339158a:/all-ruby#  env ALL_RUBY_SINCE=ruby-1.8 ./all-ruby -e 'ENV.index("RUBY")'
ruby-1.8.0
...
ruby-1.8.7-p374
ruby-1.9.0-0          -e:1: warning: ENV.index is deprecated; use ENV.key
...
ruby-2.7.0-rc1        -e:1: warning: ENV.index is deprecated; use ENV.key
ruby-2.7.0-rc2        -e:1: warning: ENV.index is deprecated; use ENV.key instead
...
ruby-2.7.1            -e:1: warning: ENV.index is deprecated; use ENV.key instead

```



-- 
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