It looks like there is a deadline now:
January 17, 2022
Chrome Web Store stops accepting new Manifest V2 extensions
January 2023
Chrome Web Store stops accepting updates to existing Manifest V2 extensions
And I’m not sure I’m reading this right but the promised in-memory storage has quota 1MB? 
Anyway, I’m still waiting for Manifest V3 support in Firefox, Safari and Thunderbird - I hope they will all make it before January 2023.