Will it be possible to migrate the older, free NIK collection into Photoshop 2021?
The older free NIK isn’t supported by DxO any longer. It is supplied ‘as is’ with no guarantees it will work, cause system instability etc etc. More like a case of licensed abandonedware.
I would suggest that you simply try it out (and report back for others who may have a similar question?) with the results.
Another related post. Seems it isn’t looking too promising unfortunately.
To be fair, the original free plugins is around 8 years old, so it has had a pretty good run.