A lot of tax preparers have posted similar questions.  There has been no technical answer but in practicality there are returns where the 8606 hasn't been filed in years or decades.  I've always taken the position that where I can, I will do a 8606 for the current year and report the cumulative nondeductible balance on that return. Of course keeping the history workpaper in my file should U. Sam come-a-calling.