These are arguments from 2016. Yes, back then CG1 was experimental, buggy and not recomendable. Since then (2016), the issues were fixed and since Java 9 (2017) G1 is the standard carbage collector. So G1 is not a experimental CG but the standard. So the 'buggy' argument is no longer valid.
Index corruption as the result of Jaba bugs: that was 2016 and if fixed. Why support G1 in Java 10+ but not in 8? I'm sure the G1 bugfixes are backported. Is this decision based on recent tests - and thus valid arguments in 2018 - or based on old data from 2016?
I think many professional parties run ES on LTS OSs. LTS OS versions typically support one major version over the lifetime of the OS. Java 11 is the first LTS Java since 8 and brand new. So it's not included in any LTS OS yet. That is why I think including offical G1 support for Java 8 by Elastic would be a big plus.