You don’t disclose assets to the IRS, only income. If you disclose your income and pay tax on it then convert it to BTC today you aren’t obliged to disclose it. In theory after you sell it you’d have a capital gain to disclose.
https://www.irs.gov/businesses/comparison-of-form-8938-and-f...
I am not a tax attorney, and more specifically not your tax attorney, nor a certified accountant.