Does metabolism really "slow down" permanently after dieting?

Partially myth, partially fact. Metabolism does typically decrease somewhat during and after weight loss, this is a real, measured effect (part of the "metabolic adaptation" response), but the idea that it's permanently and dramatically broken forever is an exaggeration for most people. The decrease is generally proportional to the amount of weight lost and tends to partially recover over time, though it may not return 100% to the exact pre-diet baseline in every case. It's a real effect, just usually less catastrophic than the more dramatic claims suggest. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/