Can a 38kHz IR receiver module (TSOP38238) decode 870Hz pulse modulation IR?
I think you worry too much about (in)compatibilities. My guess is that you can just use that TSOP38238 and that it will receive the 870 Hz pulses flawlessly.

To make sending and receiving IR codes more reliable, the sender does not send 870 Hz pulses. Instead when a "1" needs to be sent it sends a 38 kHz signal, when a "0" needs to be sent it sends nothing. This is simple amplitude modulation and the TSOP38238 can decode that, the 870 Hz pulses will reappear at it's output.

Just feed those to your Arduino so that it can decode the 870 Hz pulses into codes.

So no worries, just try it and it will work !