Doing 16:8 all year is generally safe for most people, but it's not magic - if you go back to eating like you used to, yeah, you can gain it all back because what really changed was basically the timing and maybe the amount (not your metabolism's quality). Your metabolism doesn't "break" with intermittent fasting, but it doesn't stay sped up forever either; what actually matters is keeping a calorie deficit or some kind of balance in the long run, whether it's with 16:8 or any other pattern.
If you want to keep the results indefinitely, the secret is making this a lifestyle you can actually stick with, not a temporary diet. Talk to a doctor or nutritionist if you have a history of eating disorders or metabolic issues, because then the recommendations change.