I'm very interested in this topic also.
Firstly, I agree with
@durhamlad that SS payments are only taxable in the country of residence, per the tax treaty. I've been declaring mine to HMRC and not the IRS.
Secondly, I really wasn't aware of the age 75 threshold on pensions, which is a bit of a pain. Having said that, I have my wife as the first beneficiary, so even if it were part of the estate, I assume it could transfer to her IHT-free. More complicated is if my wife dies before me, so our son(s) become the beneficiaries. If that were to happen, I suppose the best course of action is to withdraw most, if not all of it, by the age of 75. That, in itself, causes another problem because it'll then sit as cash in the UK, so will definitely be part of the estate. But it may be the lesser of two evils.