Most cell OEMs will specify safe discharge (low threshold) voltage in a datasheet. 2.75V is quite common [1].

That being said, system designer might choose higher cut-off point, since:

1) charge/discharge curve is S-shaped. There is very little energy in that last few millivolts;

2) battery (protection) circuit, and/or battery itself probably have some small leakage current. However minuscule, over months/years on a shelf, even some nano-amps of leakage will add up. If you want device to survive that, you have to factor this in, so that rest cell voltage still stays above safety threshold even after storage.

Also, "Li-ion" is quite a wide category. Don't use arbitrary voltage as a fast rule. Look up datasheet, or characterize actual cell you use. For some[2], disconnecting at 3.6V would mean leaving 50% of capacity unused. For other[3], that would be a reasonable, if somewhat conservative threshold.

[1] https://docs.rs-online.com/080b/A700000007848112.pdf

[2] https://www.murata.com/-/media/webrenewal/products/batteries...

[3] https://ntrs.nasa.gov/api/citations/20140005830/downloads/20... (page 4)

Of course you can go down there? That's literally the lower limit of design voltage?

Below 2,5V is usually when you don't wanna use them anymore