Several things that cannot be a promise seem to not have good reason to be excluded, eg

sellerExperimentGroupId
perBuyerExperimentGroupIds
perBuyerCurrencies

As async auctions become more popular, it would be useful to have as much as possible as a promise. Currently the fledge explainer lists these: "The values of some signals (those configured by fields auctionSignals, sellerSignals, perBuyerSignals, perBuyerTimeouts, deprecatedRenderURLReplacements, and directFromSellerSignals) can optionally be provided not as concrete values, but as Promises."

Is that a full list?