I filed my 2018 as always. It was simple because I had my salary 401k, mortgage, bank, and that's pretty much it. It was someday in January. I opened an account with Fidelity in March. By then, I already got my Federal tax return (and State) check back and blissfully "forgot" about that "undue torture" (reading bureaucratic e̶n̶g̶l̶i̶s̶h̶ taxglish to figure out what they want) as all normal people do. Fidelity did not "encourage" me to file 8606 and amend my return in any explicit way. If it did, it was as convoluted and implicit as even the tax law language can not be.
Again, the tax law (as I understand, maybe I still miss something) does not connect filing return and contributing to the previous year in any way. You can do it in any order. And that's why filing 8606 must be allowed in the next year. But it is not.
