Not required under law. Not required unless it's stipulated under your employment contract (very few companies practice anti competition).

hi guys, i have formally accept a new job offer and tendered my resignation

just now my current employer accepted my resignation and handed me the exit interview form

in that form, they wanted to know my new employer name, position, salary etc

is it a norm or usual practice for all employer to know where their resigned employee went to next? 

It is norm for the big organization. The reason they asked on the exit interview is because the HR wanted to know the company short fall especially when the company is unable to retain the staffs. The HR will use the data/feedback collected to negotiate with the top management.

The survey may not be doing any good to yourself since you are leaving but it will at least benefits the employers who stays or they new recruits joining the company. With the data coming from the leaving staffs, at least the HR have the bullets to fight for a better conditions with top management for the other staffs. Most of the time, with the absent of the data, the HR is unable to convince the top management that something need to be done.

By filling up the survey, at least you still keep the bridge intact in case if you want to return in the future. If you refuse to fill up, the HR cannot force you. It is up to your willingness to do some help to the ppl staying back.

yes, the HR dept already told that its not mandatory to answer it and can leave it as blank...

my situation is complicated actually, i work at company DELTA who outsourced comp ALPHA to hire me for them

i have no issues with company ALPHA, its just that there is no future of career progression in comp DELTA for me
so, its kinda sad actually to leave 

The reason companies have those fields in there is to benchmark themselves as well. Whilst you may or may not want to divulge that information, you can help your fellow ex-colleagues by putting in a ridiculous amount in there for future salary (believable) and then instead of company name, just put an industry that is similar to the company you are resigning from. If everyone does that and when the bosses review through the data of the exit interview forms, they would realise that they are underpaying their staff.

Apart from pay, some use it to find out some internal issues in the company too but unfortunately, most leavers are very reluctant to tell the truth.

tell them u work for your father or u opening a new company, sell them insurance or trust funds if they keep asking.