I paid off a coop mortgage in December of 2017. I received a 1098 for 2018 from the coop w/ mortgage interest and real estate tax. I wasn't aware I paid either in 2018.

If the mortgage company didn't credit a portion of the payoff until January 2018, that may be the reason you received the 1098.  It might be worth checking with them to get clarification.  If you itemize your deductions on your 2018 return, you would be entitled to claim the mortgage interest and taxes if it has been reported correctly.

[Edited 02.09.2019 | 4:36 pm PST]

Please see my edited answer.

but what if i had already paid the loan off and im taking a standard deduction

Again, you can check with the lender but if you're claiming the standard deduction then you won't really need to worry about it.

thank you