Would it be legal in job interview to ask if they’d repay student loan?

As the interviewee, you can pretty much ask anything you want. You may also talk yourself out of the job in the process.

I haven't seen companies offer to pay off loans before (there's no geography here, YMMV), but hiring bonuses and equity compensation are pretty standard depending on your seniority and skillset. (e.g. We'll give you $X in restricted stock over 4 years, with 25% coming at the end of year 1 and then in quarterly drops.)

At the end of the day, you're a budget line-item for the company and they care about what you cost, not what you spend the money on.

Some companies offer tuition assistance programs, which may or may not be applicable to student loans. Shouldn’t be anything wrong with asking about that benefit.

Most restrictions I've heard of are placed on the interviewer, not the interviewee.

how does loan status effect to ability to perform the duties of the job?

how would this information be used?

Biden is expected to expand student loan forgiveness in a few weeks, so it may be wise to ask for a different perk.