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Another solution which uses an ADC to read the three-state pin on the battery charger IC.  The original question mentions GPIO, and doesn’t mention ADC.  Maybe this will be useful for some other design which has an ADC.

![diagram for a level shifter between MCP73831 and a microcontroller with an ADC](https://electrical.codidact.com/uploads/9ts6abouaah6n1txgjeufvfzqf3a)

When the STAT pin of the MCP73831 is high-Z, the voltage at the ADC input is 1.6V .  
When the MCP73831 drives the STAT pin high, the voltage at the ADC input is 2.4V .  
When the MCP73831 drives the STAT pin low, the voltage at the ADC input is 1.2V .  
The microcontroller can sample this voltage with an ADC and determine the state of the STAT pin.