From the question, it is unclear that you want to alter the schema of wiki page library or want to update the item metadata columns. Since you mention Modified By or Modified Date should not update, I guess here you are working on item metadata update.

In PowerShell you can SharePoint CSOM. They key point here is to use SystemUpdate() method rather than Update() method. Read more on this at https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-server/ms461526(v%3Doffice.15)

But the downside of above workaround is it still triggers the Event Receivers and WF.

From the question, it is unclear that you want to alter the schema of wiki page library or want to update the item metadata columns. Since you mention Modified By or Modified Date should not update, I guess here you are working on item metadata update.

In PowerShell you can SharePoint CSOM. They key point here is to use SystemUpdate() method rather than Update() method. Read more on this at https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-server/ms461526(v%3Doffice.15)

But the downside of above workaround is it still triggers the Event Receivers and WF. Stop Workflow Triggering When SystemUpdate is called