Extremely unlikely. Any backdoor could be in theory found by an enemy - that's an horrible idea. As for outright prevent spare parts - these aren't systems in, well, active use. Effects would be tiny over long term.
Experts disagree with you, at least over the "long term".
>Margaret Thatcher’s cabinet secretary, Robert Armstrong, told her that “in the inconceivable case of a future US government deciding to cut off supplies of components”, there would be “no sudden effect”, though over “two or three years” the deterrent’s effectiveness would “begin to fall off”. In 2006 Commodore Tim Hare, a former nuclear-policy chief at the Ministry of Defence, told the Commons Defence Committee: “If, over a very long period, we became deeply estranged from the Americans and they decide to rat on their agreements, we would be in…great difficulty.” But the risk was “very low”.
>Sir Lawrence reckons that were that to happen there would not be a “cliff-edge” loss of effectiveness, “but over time we would be scrambling to keep it going”. The biggest problems would be replacing the Kings Bay missile storage and running out of critical spare parts.
>Effects would be tiny over long term.
The experts I quoted:
>over a very long period, [...] we would be in…great difficulty
>over time we would be scrambling to keep it going
"great difficulty" and "scrambling to keep it going" isn't "almost" the same as "Effects would be tiny "
In theory.
In reality, it took Snowden leaks for some of the smartest and most motivated people working in technology to discover they had been backdoored by a third-party.
https://arstechnica.com/tech-policy/2013/12/launch-code-for-...