As Tesla said, there is in fact no replacement for such an old microcontroller, which was discontinued many years ago. Since then, there have been an extremely large number of further developments, so that a change to a newer platform is imperative.
However, with STM8, one should not rely on a family that is not significantly younger than the ST6 and will not be available in the foreseeable future. Instead, more modern mcu such as the STM32G030K6 or STM32G031K4 would be recommended, which can easily cover the functions of the plain old ST6.
If EEPROM is necessary, one could also take a look at the STM32L010K4 because of its EEPROM technology, but due to the current extremely high demand, it is probably better to use an STM32G0 and emulate EEPROM in flash there.
Good luck!
Regards
/Peter
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