Infinite resource behavior of vanilla are both expected and not expected.

it's expressing the time to a nominal 0%, which is not possible with Factorio endless resources (they stop at 20%).

That half false, it Factorio infinite resource follow this formula for the minimum yield math.max(initial_amount / (normal / minimum), minimum) source

Extracting oil lowers the field's yield by 1% per 300 pumpjack cycles to a minimum of 20% of the initial yield or 2 oil per second, whichever is larger. source

thus this quote is only true when field is > 100% cause like you said with less than 100% yield the depletion is exponentially slower for each missing %.

100% yield is kind of a very important number for infinite resource, when yield is >100% you are guarantee to mine, for example 142% output 1 + maybe 1 (42% of chance), 284% => 2 + maybe 1 (84% of chance). That mean that with less that 100% you will maybe mine. That mean that the more close you are to 0% the more slow the yield lower.

So that lead me to my request, I think endless resource should trigger a warning at 100% so I would like an option to set a custom remaining % for endless resource.

PS: I forget to precise but endless resource percentage should probably be calculated on average contrary to current addition, say you have 7 crude oil it's should be the sum of yield divided by 7. This would give a number that better represent the real depleted progression of endless fields.

PS2: A thread where I try to gather information on infinite resource behavior