This is included in the Law against Fiscal Fraude which was published in the BOE and is valid as per 11.7.2021.
The amount used to be 2.500 euros but has now been now brought down to a maximum of 1.000 euros.
Mind this: the amount refers to the total amount paid to a professional or a business, even if the payment has been divided into several instalments that remain below the 1.000 euros limit each. This also includes wages paid in cash and refers to the brut amount, including payments for extra hours and also for meals or transport. And it is not the case that payments can remain in cash up to 1.000 euros and only the exceeding amount can be transferred, the whole wage needs to be transferred by bank transfer.
Both parties can be held accountable if cash payments are still made, but as the employee is not in a position to refuse cash if the employer insists, it is most likely that sanctions will be primarily directed at employers.