Where are you getting these rumors that the LTZ1000 is discontinued?
It clearly states "In Production" on the Analog Devices (which bought Linear Technology) website:
https://www.analog.com/en/products/ltz1000.htmlIt may be old, but there is no real need for a replacement -- it's a very good design, with no obvious flaws, and it's a Zener after all, you can't really improve much on basic physical phenomena.
There are currently only two ultra-stable, ultra-low-noise references in integrated circuit form: the LTZ1000 and the LTFLU1, the latter being a custom design for Fluke (now also Keithley, they are under the same parent corporation) and not available for general sale. And as I said, there is really no need for more, there is not much one can improve about them.
You can do the math on how much the LT1013 offset voltage, bias current, open-loop gain and noise affect the reference voltage and you will probably find it's also adequate for the job.