Installing Dolby Audio DTS plugin

Correct, it decodes it into 7.1 PCM.

I heard tvOS 11.2 was able to passthrough the bitstream signal. Say for instance I could downgrade the tvOS. Would Infuse be running on that OS?

I don’t believe that to be the true … but even if it was, you’re looking at a six-year old operating system. Highly unlikely current versions of Infuse were designed with tvOS 11 in mind; especially with Apple now on tvOS 16+. Same for all the Apple devices that were released after tvOS 11 was obsoleted.

Passthrough was available. tvOS 11.3 broke that. Infuse v5 was the current release at the time. I am not aware of how you would downgrade the aTV back to an 11.2.x release of tvOS.