This answer would be incomplete without mentioning that Dell iDRAC and HPE iLO have a lot of proprietary functionality beyond what the IPMI standard requires. For example, iDRAC and iLO support rich KVM-like screen sharing, plus the ability to mount ISOs and other media onto the server. Indeed, so much more functionality exists in these implementations that a license key must be purchased to enable the most fancy features.
I will note that SuperMicro does simply call their offering as “SuperMicro IPMI” despite having a few of these proprietary features. But by and large, basic IPMI is an interoperability specification, with each implementation having their own unique strengths.