What has to be on a payslip?
The law requires an itemised pay statement for every employee and worker, given on or before the day they are paid. It must show gross pay, each deduction and what it is for, and net pay. Where pay varies with hours worked, the number of hours must be shown too.
In practice a payslip also carries the tax code, the National Insurance number, the pay period, the year-to-date totals, and the employer's pension contribution, which is not a deduction from pay but appears for completeness.
It can be paper or electronic, and most now are. What matters is that the employee can get at it — a payslip that lives only in a portal they lose access to when they leave is a common source of P60 disputes.