If a single retired person dies and they don’t have a will or assets and nobody claimed them as a dependent, does anyone have to contact the IRS?

[jdeans31]

Is anyone responsible for filling their tax return?  

[Vanessa A]

Their next of kin or whoever was handling their final expenses would be the person that would be able to filing their final return.

Death in Family

[Opus 17]

If a person died in 2023, someone would need to file a 2023 tax return for them if they have taxable income.  If their only income is social security, that is non-taxable, but if they have investment income, properties, interest and dividends, pension, IRA withdrawals, etc., they might have income that needs to be reported and taxed.

(However, apart from the requirement to file a return if they have taxable income, there is no separate procedure to inform the IRS that someone has passed, if they aren't required to file a return.)

Someone must handle their final affairs.  If there is no family or friends to do it, maybe the state takes over.  You would need to check the laws in your state.