The LTZ1000 thermal modelling I looked at on eevblog does not include the glass on the Kovar leads.
Example Schott 8250 borosilicate is 1.2W/m-K so a good insulator.
I expect the LTZ leads would be supplied heat through that high thermal resistance (TO-5 base-glass-lead), meaning PCB pad temperatures even more sensitive than models suggest, to slots, airflow and copper traces.
I've stopped working on this as others have done 3D modelling, compared to this limited 2D work which can be misleading.
edit: about slots on
SMT packaged references from
AN82 Understanding and Applying Voltage References:Board Stress
Burn-in can help “relax” a stuffed board, but additional mechanical stress may be introduced when the board is mounted into the product. Stress has a directly measurable effect on reference output. If the stress changes over a period of time, it may manifest itself as unacceptable long-term drift. Circuit boards are not perfectly elastic, so bending forces may cause permanent deformation and a permanent step-change in reference output voltage.
Devices in metal (TO-5 and TO-46) packages are largely immune to board stress, owing to the rigidity of the package and the flexibility of the leads. Plastic and surface mount packages are another matter.
Board stress effects are easily observed by monitoring the output of a reference while applying a bending force to the board. A controlled experiment was performed to measure the effect of board stress on an LT1460CS8-2.5
surface mount reference; see pic