@nitrofurano the OPL4 expansions were not made for the MSX during its lifetime, and did not have a fixed specification, with different models having differents amount of Sample RAM and etc.
Also my proposal for the format avoided overlaps with other existing formats, hence SN76489 and YM2151 not being included, despite having expansions with it at the time (YM2151 being built-in in Yamaha machines). It's also not on the
.KSS specification which includes these 2, so the only song format it would share with msxplus would be .VGM and .DSK, at which point it can bring more questions of more homebrew expansions since you can hook anything to an MSX. Becoming just another VGM-like format.
There's also the subjective matter of the OPL4 just not being a chip that heavily associated with the MSX in the first place like the ones accepted here are. I think
OPL4 should be its own format.
Edit: edited the lyceum to explain the expansions and the reasoning of their choice