Crash diets

I'm tired of people on the internet saying "crash diet bad because bad" and want an actual clarification.
From what I have gathered:

When doing a crash diet, after a week people lose water weight, think they lost fat and immediately go back to their old diets without any exercise making them quickly build more fat.
Because the regular person is a fucking slob, and it seems everybody only talks about the regular piece of shit who isn't up to actually bettering themselves and only wants a quick fix.

What would happen if I did a crash diet for long enough that I got completely rid of undesirable fat (20KG), and then when I'm done, since my metabolism went starvation mode, I maintained a normal but healthy diet while doing an insane amount of cardio and always staying at a caloric deficit? Would I still rebound? What would be the reason then?

TL;DR: crash diet + insane caloric deficit when done = fat not come back?

Would be nice if you had a source.

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brother just eat every other day and eat one days worth of your maintenance calories on the days you do eat. ideally eat one or two meals on your eating days, eating 1200 calories everyday is like edging to porn

or you just live like a normal non-autistic person

how about you actually eat what humans are supposed to fucking eat and you wont have to diet just stop eating man made food its literally that simple

i literally eat as much meat/organs,eggs and fruit as i want like 4-7k calories a day and im still not fat after 2+years

Can't because gastritis. Thanks for the advice tho.

You're still not fat because you weren't far in the first place, I need to lose weight.

either fast a day every now and then while not overeating the rest of the days or eat just your protein requirements from almost lean meats say 100g prots and 40g fats known as protein sparing fast or something like that
dont do the meme diet of fruit only that women share on insta

How am I supposed to live off of 140 grams of food in the day?

>Would I still rebound?
yes because you CAN'T maintain insane amount of cardio like you're planning to forever. i know it sounds gay boomer but the only way is to do a lifestyle change and chip fat away slowly. good news is you lose it only half as fast than you put it on on. only.

t. been there done that

I AM NOT INTERESTED IN YOUR SHITTY DIET RECOMENDATIONS. I MADE A SIMPLE QUESTION AND I WILL REPEAT IT.

TL;DR: crash diet + insane caloric deficit when done = fat not come back?

Not maintain it forever, only until my metabolism leaves starvation mode. Then I do normal cardio. Why would I still rebound?