I can only tell you what I know from my own experiences. I seem to recall some members on another thread mentioning that as long as equipment had been removed from your account for not longer than 6 months, it could be added back. However this was in relation to Superhubs, so whether the rules differ or not for TV set-top boxes I do not know.
My experience was I 'upgraded' from V+ to TiVo in September 2016. It was slow, unreliable and generally horrible to use. After CS agents promising an update was pending to resolve the issues (was lied to basically) I initially kept it. By February/March 2017 (can't remember exactly when) I was thoroughly fed up with it but due to their lies I was now well out of the 14 day cancellation window to revert back to my previous equipment. I phoned them and told them I wanted to go back to V+. Was told no initially, but after several hours of phone calls spread across multiple days, they did it. And they must have been able to see it on their screen because they didn't ask for the box serial number or anything.
So yes equipment no longer on your account CAN be added back, but if it's been inactive for some time (over 6 months) then maybe not.
Edit: And by the way, it was Retentions that were giving me the run around and being obstructive. It really is pot-luck when you ring up whether you get someone that a) isn't an idiot and b) actually wants to help. Generally off-shore are a lot worse but they can all be bad at times.