Commonly this is being asked for Form 8606 related to IRA distributions or Roth conversions etc, this value is required by Form 8606 line 6 to calculate the correct tax if you have a basis in the IRA and the pro rata calculation applies. The verbiage from TT is misleading - as you say 5498s are usually sent out in May (and would provide year-end MV), instead you should use your year-end brokerage statements as per 8606 instructions below. I'm not sure if this question is asked elsewhere but if so there will likely be similar IRS instructions.
https://www.irs.gov/pub/irs-pdf/i8606.pdf
Line 6
Enter the total value of all your traditional IRAs as of
December 31, 2025, plus any outstanding rollovers. A
statement should be sent to you by January 31, 2026,
showing the value of each IRA on December 31, 2025. (etc)
