A friend, retired teacher, reports a $16,000 deposit from Social Security for retroactive 2024 benefits.  

The question asked, of course, is whether an ES payment should be made now, or just wait until the 2025 return is filed.  As long as 2025 withholding equals 2024 tax (or, if applicable, 110%) I don't see any reason not to earn 4% on it for more than a year.  But if taxpayers are worried about it burning a hole in their pockets, there is no law preventing them from paying it forward.