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