Fully Migrating Exchange Server 2019 to O365

TL;DR
For ease of management, it might be beneficial to keep the latest version of Exchange in Hybrid config and keep it up to date all the time.

Here are several links that support the recommendation to leave at least one Exchange hybrid server on-prem:

Azure AD Connect and removal of on-prem Exchange 2010 server
https://docs.microsoft.com/en-us/answers/questions/9221/azure-ad-connect-and-removal-of-on-prem-exchange-2.html
 
Removing Your Last Exchange Server FAQ
https://techcommunity.microsoft.com/t5/exchange-team-blog/removing-your-last-exchange-server-faq/ba-p/3455411
 
How and when to decommission your on-premises Exchange servers in a hybrid deployment
https://docs.microsoft.com/en-us/exchange/decommission-on-premises-exchange
 
Decommissioning your Exchange 2010 servers in a Hybrid Deployment
https://techcommunity.microsoft.com/t5/exchange-team-blog/decommissioning-your-exchange-2010-servers-in-a-hybrid/ba-p/597185

Office 365 and Dirsync: Why should you have at least one Exchange Server on-premises
https://mymicrosoftexchange.wordpress.com/2015/03/12/office-365-and-dirsync-why-should-you-have-at-least-one-exchange-server-on-premises/

Exchange Server Roadmap Update
Exchange Server Roadmap Update | Microsoft Community Hub


Some of the links might be talking about older Exchange versions, but from perspective of Hybrid config that does not make a difference. You can remove Exchange and manage everything through either scripting or ADSIEDIT, but that is more on the side of management, not functionality.