What should I do about my landlord's expired ID for my retirement extension visa in Thailand?

And I guess the land office or a lawyer are better options than this page as the next of skin and property owners, lease contracts and payments has nothing to do with Thai visa advice.

But if you like to know the answer, take the expired documents, hand it over to the Immigration Officer, explain that valid documents are not possible due to death and you will get an answer.

I presume there’s a new landlord for the building .

immigrations won't have any idea that your landlord is dead, use the old lease and what you have. At least that's what I'd try to do first and see if it flies or not.

to whom do you pay the monthly rent ? his/her ID should be the document you require

When peole die, they stop to be an active member of the society and therefor can not be used as landlort or guarantor. You have to find the new land lord, they one who comes by to collect the rent might know more