> Librem uses a very proprietary, very closed SoC.

That is FUD. Yes, it is proprietary/closed. To say that it is _very_ proprietary and _very_ closed would imply that it is somehow _more_ proprietary and closed than normal. However, the higher quality documentation available for the i.MX series would make it any more closed. It is ever so slightly more open.

>FUD

Very was meant to highlight the contrast with a SoC that has open RTL.

i.MX is indeed one of the better documented ARM SoC families.

Is there any good source to read more about how closed it is?

Um, you have the burden backwards; closedness means a lack of documentation.

If you want to try, go here:

  https://www.nxp.com/products/processors-and-microcontrollers/arm-processors/i-mx-applications-processors/i-mx-8-processors/i-mx-8m-family-armcortex-a53-cortex-m4-audio-voice-video:i.MX8M?tab=Documentation_Tab

And try to download the "Security Reference Manual for i.MX 8M Dual/8M QuadLite/8M Quad". You can't. It's only available under NDA to extremely high-volume customers. And that manual is where all the goodies are.