BTC ETF in in a 401K/IRA can be contributed, grow, or possibly be withdrawn tax-free depending on how you set it up. Coinbase reports your gains and you owe taxes on them. (Don't think Coinbase has an IRA/401K option for custody. Yet.)
I was referring to the capital gains tax you pay for selling BTC on an exchange. I was thinking specifically about Roth IRAs, where any gains are tax free, but a 401k or similar would work too.