We support ordinal color scales:

Plot.legend({color: {type: "ordinal", domain: d3.range(10)}})

But trying to do the same with an opacity scale results in an error:

Error: ordinal opacity scales are not supported

Plot.legend({opacity: {type: "ordinal", domain: d3.range(10)}})

I see no reason we couldn’t support ordinal opacity scales by dividing the range interval [0, 1] equally based on the cardinality of the domain.