Greetings, welcome to the Forums.
I agree, the plant pictured in the OP is basically dead. It is not clear what caused the initial decline, possibly either underwatering or overwatering, but removing all the soil from the plant and soaking it for several days in a vat of water was surely its death-knell.
That was not at all the right thing to do.
If overwatering or poor drainage caused the decline, then reduced irrigation and/or correction of any drainage problems with the soil, contianer or saucer needed to be done.
Sometimes a container plant's soil becomes overly dry, and soil becomes hydrophobic. it will then need to be soaked to allow water retention to occur. This is done by soaking the entire container and soil rootball of the plant in a large vat or bucket for a few hours or overnight. If the container and rootball is so light that it floats, it must be somehow held or weighed down underwater so proper soaking can occur.
Rosemary (Salvia rosmarinus, formerly Rosmarinus officinalis) grown in containers should be planted in well-drained potting soil, watered regularly and given full sun conditions. Growing Rosemary indoors is not ideal, but it can sometimes work in very sunny/high light settings.
Planted in-ground, Rosemary can grow well in full (to part) sun, in clay, loam, or sandy soils, but drainage must be excellent and warm-season irrigation should be low to none. I grow Rosmary in well-drained clay soil, in full sun, where it gets virtually no summer irrigation.