Yes. Its also been added into various LTS versions about five or six months ago.

https://cdn.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.16....

https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.9.1...

https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14....

https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.19....

ctrl+f: cpu/speculation: Add 'mitigations=' cmdline option

it is, but I know a fair amount of people on older distros - ubuntu 16.04, 18.04, etc... whom may have some flags available but not mitigations=off which is a more recent addition

One of the article's comments says so.