docs: add deprecation warning

6 jobs for master in 117 minutes and 31 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #625715
macos_highsierra
test-macos

00:17:22

 
  Build
passed #625716
build-latest

01:38:28

passed #625717
build-nightly

01:18:28

 
  Docs
passed #625718
docs

00:01:17

 
  Deploy
passed #625719
pages

00:00:23

passed pages:deploy

00:00:01