in 2018 there was a hope of siacoin in coinbase custody: https://thenextweb.com/hardfork/2018/12/07/coinbase-cryptocurrency-list-altcoin-xrp/


and now seem that siacoin is no more considered : https://thenextweb.com/hardfork/2018/12/07/coinbase-cryptocurrency-list-altcoin-xrp/