The only reasonable way to avoid backdoors is to meticulously check the submitted code.
Which is the job of maintainers. Which now aren't Russian, any more. To the best of my knowledge the kernel is still accepting code from Russian citizens, ultimately not having Russians in maintainer roles isn't going to stop the FSB from infiltrating the kernel but it certainly does make it harder.
This also isn't in any way a judgement on the removed people, it's just that it so happens that if you're a Russian citizen you're quite vulnerable to wrench attacks. You could even say that the kernel org is protecting them from being used like that.
FSB wants backdoor in kernel. FSB notices subsystem maintainer is Russian, lives in Chelyabinsk. Can close eyes to backdoor, can pretend to review. FSB in Moscow make call to FSB in Chelyabinsk telling to buy heavy wrench at hardware store.