Is there a pending or publicly discussed legislative proposal to impose a 20% excise tax on defined benefit lump sum distributions?

Might be; have not heard that one. You might tyr a search at Thomas

I have heard of proposals to exempt the first $X (such as $2000) from federal income tax if that is part of an annuity. Probably very little chance for passage since it is a direct "cost" to tax revenues.

Even if they did enact such a tax, which is unlikely, my guess is that you would be able to avoid it simply by rolling over the whole amount to another tax-qualified retirement plan or IRA.

But I wouldn't be surprised if they raised the tax on premature distributions (received before age 59-1/2). I have been expecting that to be enacted for some time.