It's worth nothing that unlike in 2012, the last time New Zealand took part in the men's Olympic tournament, clubs are under no obligation to release under-23 players for Tokyo 2020.
It's also worth nothing that the OFC Nations Cup will be held earlier in the (European) summer, which will throw a spanner in the works for A, over-age players and B, under-23 players also in the All Whites frame, especially those at European clubs and American clubs (those at A-League clubs, who have an endless off-season, not so much).
Edit: For the sake of absolute accuracy, Fifa hasn't said actually said anything re: release of players for 2020. But in 2016 it was 'please release them because it's cool, but you don't *have* to'. In 2012 it was 'you have to.'.