Yes, it is possible. Your AD schema was extended with Exchange attributes. As long as all schema extensions are permanent, even without on-premises Exchange the attributes will be available for management in Adaxes as any other AD attributes.
At the same time, you will not be able to use the attributes for mailbox management. For example, if you keep AAD Connect working for Exchange properties and make any changes to a mailbox in Microsoft 365 (former Office 365) on the next synchronization your changes will be overridden by the values of on-premises attributes. That is why Microsoft recommends to properly decommission Exchange servers in such cases. For details, have a look at the following article: https://docs.microsoft.com/en-us/exchange/decommission-on-premises-exchange.