What is the approximate battery discharge rate of Google Pixel 3XL phones when turned off? I am considering purchasing a new Google Pixel 3XL Android phone, primarily for photography.  I don't believe those phones have been manufactured for 2-3 years, so I am concerned about the battery health.  If the phone has been sitting in a box for 2 years with 0% charge, my understanding is that it's likely the battery will not function well.
For this reason, I'm wondering: What is the approximate battery discharge rate of Google Pixel 3XL Android phones when turned off?
(Also, if anyone happens to know how approximately much battery charge was present when the phones were placed in their box, that will be helpful.)
The answer starts from what you asked towards the end of your question.

Li-ion battery's are charged to around 40% while storing. That's about the charge level (give or take few % points) you'd see when you take a phone out of box for first time. Reason


Lithium-ion must be stored in a charged state, ideally at 40 percent. This prevents the battery from dropping below 2.50V/cell, triggering sleep mode.

Sleep mode is a protective mechanism to prevent deep discharge and you can't charge the battery conventionally when it enters that mode.

But batteries do self discharge and the typical rate of Li-ion batteries is about
2% per month (Table 3). But this does not mean you get a fully discharged battery say after 2 years. That's because


There is virtually no self-discharge below about 4.0V at 20°C (68°F); storing at 3.7V yields amazing longevity for most Li-ion systems


But yes, your battery would suffer a capacity loss as you can see in Table 2 of link quoted in above point. If you can get a new battery from OEM, which is manufactured recently, this risk can be mitigated and you can use it for long.
But if your use case is only photography, I guess you need not change battery at all.

This is generally applicable to all Android devices and  your specific model should not be an exception.