Aspeed aren't the ones keeping them closed - OpenBMC running on Aspeed chips is an option chosen by some system vendors. It's a vendor choice whether to go with an open platform or a closed BMC from AMI etc.

https://github.com/AspeedTech-BMC/openbmc (or see the upstream OpenBMC tree)

HPE are adding some OpenBMC support https://www.hpe.com/us/en/compute/openbmc-proliant-servers.h...

This work from Supermicro looked promising https://lore.kernel.org/openbmc/CACPK8XdE0sRmt4x54YJVJO2wDT5...