Since it was a 2017 contribution, it must be reported on a 2017 tax return, but you will not receive the 2018 1099-R until next year since it was removed in 2018. The 2018 1099-R would have a code "P" in box 7 meaning it is taxable and reportable on your 2017 tax return.
You do not need to wait for a 1099-R next year that will probably require you to amend your 2017 tax return to report the excess on line 7 of your tax return - it can be entered this way and then ignore the 1099-R when it comes.
Excess 401(k) deferrals should be reported in:
(There are several screens to click through to get to the right place)
Miscellionious Income ->
Other Income not reported on a W-2 ->
Other wages (yes) ->
House Hold employee (Continue) ->
Sick Pay (Continue) ->
Other earned income (yes) ->
Source of income (other) ->
Any other income - enter the amount of the excess deferral and an explanation.
For information see IRS Pub 525 page 10
https://www.irs.gov/pub/irs-pdf/p525.pdf
Note that the IRS Pub refers to 2016 & 2017 excess reported on the 2017 tax return and does not discuss 2018 1099-R's because they have not been issued yet.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
