SST is not paid to IRS

One thing is for sure that SST is not paid to IRS. Therefore, it won't be IRS on your back. But frankly, from legal point of view, I don't know if there are any other consequences.

I guess since you didn't pay for the past year, your eligible social benefits will be missing this part when you retire as the Social Security doesn't have a record for you yet. You should notify the employer to start withholding SST for you immediately.