Pro-tip: If you're a Xenophobic Necrophage in 4.3 and have species set default to necropurge you ignore the stellar culture shock penalty even if you disable it after.

So you can invade planets and either purge completely or let it run until you're happy and decide you want to save some pops, and never have to deal with the negative modifier at all.

I'm pretty sure it automatically removed itself after the first new month tick, but after that what happens to the pops is up to you, and it's much better than being forced to deal with the drawbacks for a decade.

😉