Hi @Aditya Biswas ,
Thank you for posting in this community.
Unfortunately, the full Track Changes feature is not available in Office Online for SharePoint 2019.
The track full changes feature is not available in the web interface by default. It is typically a feature that is available within the Office desktop applications like Microsoft Word.
If you want to enable track changes for documents in SharePoint, you may need to use the desktop version of Microsoft Word to open and edit the documents from SharePoint. Track changes can then be enabled within Word.
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