Appears to me to be increasingly too much shade from surroundings over the years.

More likely something adverse in the rooting environment, like water molds, honey fungus or - perhaps even nematodes are possible, I don't know (not a big factor in my area).

To answer the "is it dead?" question (which seems to have been skirted around) - yes, it is. PIttosporum is an evergreen shrub and when an evergreen shrub loses all its leaves, it's a pretty good bet the plant is dead.

Except when what happened is most of the top froze off in a cold spell and the type of shrub is one that can grow back from the crown.

Is there a recommended type of soil for this plant to stay healthy?

It doesn't really need anything special in the way of soil. Your native soil should be fine as long as it is well drained and isn't soggy. But it does need good light for the best colour.

In my garden they don't like shade, and they need regular water. Do not amend the soil. They are best in Sunset 24 and 23. The farther inland you go, the more they struggle.

'Marjorie Channon' does fine here in Sunset zone 16.

There seems to be a rather high incidence of P. tenuifolium (of which Marjorie Channon' is a variegated cultivar) succumbing suddenly to what is suspected to be a root rot. Phytophthora is the suspected culprit although several other similar soil pathogens have been found to be common as well. Not saying this is what happened to the OP plant but is certainly a possibility. Apparently there is a very fine line between under watering these plants and over watering. I grow mine in a container with a fast draining soil mix and so far, so good at 3+ years.

I'd also check any dead or dying plants for root knot nematodes, which can produce very similar symptoms and dieback.

I've planted 16 of them just a two weeks ago into 50/50 mix used here for lawn grass. I don't have any special drainage, just 11" x 30 ft. in-ground sections filled with this soil. They've all turned brown started dropping leaves within 3 weeks. Not sure I've over watered them on the first day. I did fill up holes with water prior to planting.