Is a thrombus in carotid life threatening?

Yes. Thrombus is clot. Most carotid disease is detected as plaque thickening the wall of the artery, and if severe can send small emboli (clots) to the brain causing a stroke or ministroke.The only frank carotid thrombosis I have seen was one day after endarterectomy, eating breakfast one minute, not talking the next. Prompt re-operation brought her back to full function, no deficits.
Stroke Threatening. Atherosclerosis, in the artery walls, starts in childhood, asymptomatic for decades because artery wall thickens & enlarges. Most events & narrowing due to plaque rupture & clots. Primary driver lipoproteins (not cholesterol), blood sugar & BP + others. However, once clot present, high risk of stroke. At the same time most are missed if no symptoms occur & scar down, rarely near totally resolve.