You should use the Social Security Number (SSN) you have used in the past to file your income tax return.
On the one hand, your Social Security Number (SSN) is yours for life. It will not be canceled or changed if and when the DACA program truly ends. (There's still a chance that Congress will pass legislation creating benefits similar to DACA.)
What's more, the amount your employer deducted from your paycheck and paid into the Social Security system remains on your record, and may create a possibility of benefits for you when you retire or are disabled.
On the other hand, having received your Social Security Number (SSN) through the DACA program, your card probably contains the following text: "VALID FOR WORK ONLY WITH DHS AUTHORIZATION." (A Social Security Administration site).
The IRS would not directly exchange information with your employer. Your employer would be reporting your wages on a W2 that is submitted to the IRS exactly like the one they provided to you.
Please review more information here if you like: Frequently Asked Questions on DACA Termination
