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OnePlus update blocks downgrades and custom ROMs by blowing a fuse
(consumerrights.wiki)
This is a most excellent place for technology news and articles.
Yeah, this was the case shortly after they release of model 5T or around thst time. That's when they stopped being the only company that would provide instructions to root your device, and guess what... Back then, if you rooted your device the warranty was still valid!