docs: add deprecation warning

6 jobs for master in 74 minutes and 25 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #629895
macos_highsierra
test-macos

00:17:08

 
  Build
passed #629896
build-latest

00:55:37

passed #629897
build-nightly

00:43:16

 
  Docs
passed #629898
docs

00:01:09

 
  Deploy
passed #629899
pages

00:00:29

passed pages:deploy

00:00:00