There are two primary factors
General internal resistance of the cell. This is the current the cell naturally consumes itself due to entropy and its own internals not being a perfect insulator for the various sections within. This is also the thing that people note when they say batteries don't last as long in the cold.
Quiescent current. AKA the power the device consumes when "off" to keep clocks, RAM and whatever else the manufacturer deems necessary powered and I doubt they have a "not yet set up and in a box" mode.
Similarly it is not 0% you are necessarily worried about here; deep discharge is the position, or more like range, you are concerned with. Many would equate it to 0% on a screen though (which is something of an arbitrary position but usually set a bit above the point where deep discharge has set in). Some charge management will be able to bring it back from this (it will have some cost in capacity, though how much depends on a few things), some usually cheaper ones will not but by 2019 then it was pretty common to have the features compared to earlier chips.
I can cover some boring chemistry if you like but I will leave it at that for now.
I could give you a decent model for the former if knowing average temperature for the area/that it was presumably kept, with some leeway for whatever they shipped it as (max charge both takes a long time over say 70%, adds up in power costs/manufacture time if doing hundreds of thousands and is not necessarily the best thing for storage purposes), for the latter I could measure it with some equipment but I doubt many reviews or specs anywhere will have noted it (I would be somewhat pleasantly surprised to find 2 models of iphone or flagship android, not random phone like this).
On this deal is it sold as seen or could you return it if it does not work? Even if it means sitting in the shop while it charges to see*. Having it work at all and lasting more than about 30 minutes would be my bigger concern here. If it makes that and has not gone puffy** then chances are it will probably be a better battery than someone that spent the last two years using fast charge.
I have certainly had used lithium battery devices be pulled out of a box after a few years and charge up just fine (would happily buy, and have happily bought, any number of handhelds that were presumably put aside and then donated to somewhere or sold off cheap and functionally they are similar devices).
*have a nice sob story about family coming into town and "it needs to work" if you want.
**will be fairly obvious for any design of phone I have ever seen. Either front panel or back will not be flat where it once was.
