On a Microsoft blog it states that WordPad is no longer being updated and will be removed in a future release of Windows.

"We recommend Microsoft Word for rich text documents like .doc and .rtf and Windows Notepad for plain text documents like .txt."

While Microsoft Word might be heavy  to fill the WordPad role, RTF is rarely used these days, and anyone can access the web versions of Word for free if needed. In fact, we can't remember the last time we used WordPad. 

When it came out it was much better than Apple's word processing software, but already behind Word.

The actual date of removal is unclear, it will probably be about the same time it kills off its old Transport Layer Security protocols to make Windows 11 more secure.