What does OnePlus have to do with this? I'll admit I'm not down in the trenches on ROMs, etc. but my understanding from what I've read over time is that it runs like this:
CyanogenMod is a popular and widely used ROM
Core developers of CyanogenMod and possibly some business folks decide to turn it into a company and provide said ROM (with customer-desired tweaks) to small hardware vendors who don't want to do their own in-house development
Two of the client companies are OnePlus and someone in India, but contract screwups mean that because of this OnePlus can't sell their phones in India. Displeased with losing a huge market for inexpensive phones, they drop Cyanogen and go their own way for future products.
Further drama and possibly not enough sales ensue
Internal drama at Cyanogen leads to the founder of CyanogenMod leaving
Cyanogen the company shuts down.
I'm sure there's a ton of nuance, hurt feelings, betrayal, etc. involved (aka "drama"), but blaming OnePlus for Cyanogen shutting down makes no more sense than blaming Cyanogen for OnePlus shutting down if they'd gone under due to the whole "can't sell in India" thing.