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.'.
Are you sure of that? Generally speaking there's no obligation on clubs to release players for any age-group tournaments, including the Olympics, why was 2012 different?
https://global.espn.com/football/story/1042541/fifa-rule-clubs-must-release-players-for-olympics?src=com and many similar reports.
This letter from 2008 suggests Fifa always intended that to be the position: https://www.fifa.com/mm/document/affederation/administration/1153-releaseofplayersforthemen%E2%80%99solympicfootballtournamentbeijing2008_58691.pdf
There was later a shark-storm in 2008 about Messi (under-23 at the time). The court of arbitration for sport ruled Barcelona didn't have to release him because Fifa didn't list the Olympics in their international tournament calendar.
It appears that was what prompted Fifa to come out and make things black and white in 2012.
Not sure what had changed by 2016 (https://www.fifa.com/about-fifa/who-we-are/news/fifa-executive-vows-to-improve-governance-and-boost-female-participati-2696406). My guess would be pressure/lobbying from European clubs.
As I noted with the edit, Fifa might very well come out and say it's back to the old ways for 2020, but I doubt it.
Edit: I don't think it's *that* big a deal – but the likes of Bayern and Willem would probably not be too pleased having Singh/Woud late starting their off seasons with the Nations Cup then away during pre-season for the Olympics, so might force them to choose one or the other.
