docs: add deprecation warning

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  Test
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macos_highsierra
test-macos

00:17:43

 
  Build
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build-latest

02:01:19

passed #628466
build-nightly

01:46:51

 
  Docs
passed #628467
docs

00:00:51

 
  Deploy
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pages

00:00:15

passed pages:deploy

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