If i stop taking thyroid replacement therapy after an ablation will i die?

Possibly: If your thyroid is completely inactive, then you do not make any thyroid hormone which is necessary for life. It would take a long time, and the changes would be quite slow (fatigue, wt gain, hair loss, decreased body temp, etc.), but eventually, you could experience myxedema coma which is life threatening.

You can: You can, but you will get very sick first.