How to make processing go.sum in 1.16 as in 1.15

there are major changes in 1.16 in processing of module mode, I have some projects which tuned for 1.15 and trying tune them to be built under 1.16. It is possible to restore auto module processing through GO111MODULE=auto for projects which don’t use modules but projects which use modules stopped their build with complains about missing go.sum (for external dependencies). How I can restore processing of missing go.sum in 1.16 to get its work like in 1.15? I was trying to playw with GOSUM and GONOSUM but without success and on level of code it looks like that in 1.16 such behavior just hardcoded.