You cannot suppress Recently Closed Tabs without deleting Safari history.
Recently Closed Tabs are intricately linked with Safari history, and clearing Safari history entries deletes: (1) the tabs, and (2), the Recently Closed Tabs «history».
Furthermore, there is no other option than deleting Safari history to delete recently closed tabs in Safari or in Settings.
This is one of the rare issues with Safari and I hope there will soon be an option to make users able to clear that recently closed tabs «history».
I just found a way to delete the most recent closed tabs.
This only works in custom Tab Groups.
You need to create a Tab Group (or open one if you already have some).
Then, it should open the Tab Group on Start Page (Safari's home page). Click on Edit and make sure Recently Closed Tabs is enabled. Now, click on the "X" at the top left of the "Customize Start Page" dialog window to close it.
When you will close a tab, it will show up in the Recently Closed Tabs section in your Start Page as well as in the Recently Closed Tabs (when you long-press on the + button next to the address bar).
Before clicking on "Clear All" :
Then, click on "Clear All", this should make the "Recently Closed Tabs" section in your Start Page disappear. But, it will also make the elements in that section also disappear in Recently closed tabs (when you long-press on the "+" button)!
After clicking on "Clear All" :