My first employer was a startup and not very keen on documentation. And so I did not receive any salary receipts. My new employer is demanding the salary receipts from the previous employer and said I can show my bank statement if I don't have those. As I understand, my financial transactions are not concern of the new employer. In that situation, can I strike off (cover with ink) other transactions leaving the salary credit? Is that an accepted practice?
Yes, you should be ok to strike off any visibility of financial information that isn't related to your previous income (if it's legal for your employer to ask).
Your new employer only needs to see the amount of salary paid and verify the payer reference. If they need to see anything more than that, then you'd really have to question that request.