Is it true that everyone gains all the weight back within a year of stopping?

Mostly fact as a general pattern, though "everyone" and "all" overstate it slightly. Clinical trial data has shown weight regain is common within about a year of stopping for most people, though the exact amount regained varies by individual. Some people regain most of it, others regain a portion while retaining some of the loss, particularly if meaningful lifestyle changes were sustained. It's accurate to say regain is the norm rather than the exception, just not universally 100% for every single person. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/