Before GOES 17 took over as GOES West, GOES 15 was the previous GOES West satellite. It is still in orbit and it is effectively on standby in case there are issues with one of the current satellites, GOES East (GOES 16) or GOES West (GOES 17).
With the issues that GOES 17 has had with keeping its image sensor cool and the more recent outage due to a memory issue, having a standby is even more important.
GOES 15 gets a semi-annual check out where it is capturing images and some channels can be picked up as they are shared via GOES 17.
The latest images can be found on the GOES 15 page, however outside of these short tests they may be a bit outdated.
Some of these images look a bit different to the usual images we get from GOES 16 and 17.