If you cant upgrade then users can use Outlook Web App.
You can still use Outlook 2010 however if you want. All it means is its not supported and out Office 2010 is out of support as well so if something doesn't work, you won't get support to fix it.
about office 2010 with exchange 2019

Chinmoy Das
41
Reputation points
Hi all, I recently installed the microsoft exchange server 2019.I would like to know if Office 2010 will support as clients in this environment.
Outlook | Windows | Classic Outlook for Windows | For business
10,157 questions
Exchange | Exchange Server | Management
7,925 questions
Accepted answer
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Andy David - MVP 158K Reputation points MVP Volunteer Moderator
2021-12-10T12:41:56.533+00:00
1 additional answer
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Andy David - MVP 158K Reputation points MVP Volunteer Moderator
2021-12-10T12:14:03.29+00:00 Hi, Outlook 2010 is not supported with Exchange 2019: