You can still use BCS with SharePoint Online but because of the lack of some features, and the difficulty in getting the connections to work with permissions, people switched to using PowerApps instead.
Here is a good article if you have to use BCS. https://aqltech.com/connecting-sharepoint-online-with-bcs/
Keep in mind SharePoint searching is not available using BCS so you would need to setup some fancy filtering instead.
Hope that helps,
Kristopher