Edit: The OP is very convinced that this is not an XY problem so I will directly address his comment below:
This answer is a bucket full of red herrings. Again, my question was: whether deducting W2 FICA Employer match (from my pay) is legal from the IRS' perspective. I don't see an answer here.
The answer is that "it depends" and "your accountants will hate you." Generally, your employer can deduct any sort of fees you agree to in your contract; so, the starting point is that you are free to contract and work under those terms.
There are knock on effects that would make it illegal, for instance using this as a way to pay less than minimum wage would not be legal. Not informing you about it or retroactively imposing this as a policy would be illegal.
The accounting implications are also significant, this is just a scheme to give the government more tax money. Lets say you make $100 in a normal W2 situation. You are costing the company $107.65 with their FICA contribution and they are sending $15.30 to Uncle Sam. If you move that $7.65 into your pay then the taxed amount is not $100 but $107.65, so government would get $16.47. That also now means there is still an "unpaid" employer contribution left of $1.17 which if you iterate converges with a total cost of $16.68.
I did that math assuming the employer would be increasing your wage to cover their contribution for simplicity, I am sure it would be going the other way. Regardless, by paying you and then charging you the employer contribution the end result will be a FICA tax bill that is about 1.3% higher.
The other bit of tax implication is that normally the employer portion of FICA is not taxed. But now you will also be taxed on that as income with effectively reduces your income by 7.65 times your marginal rate (say 22%) so that will also increase your taxes by another 1.7% or so.
This scheme results in a tax bill that is $3 more an hour, is non-standard, and complex. To bring this topic full circle this is why I think you asking an XY-problem type question, the way you are suggesting you are being cheated doesn't make any sense financially. It does make perfect sense however if you are looking at the way the company calculates equivalent cost 1099 vs W2 rates.
Summary: Not illegal when negotiated as part of your contract while being financially counterproductive and generally dumb.