How can I read .rtf file with Safari on my iPhone 11 Pro?

Safari is a web browser. I wouldn't expect it to read a .rtf file.

However, your phone should support Pages, a word-processing based application that can read and process .rtf files. Therefore your best path is likely to have Safari download the .rtf file to your Files, and then open it in Pages.

You can ignore the features you don't like... but if all you want to do is view the file, then download the file to your phone and find it in the Files app... from here a long press on the file displays a pop-up menu, one of the options here is Quick Look.



This will open the file and render it so you can view it - no additional software required.

As its name suggests, you can only look at the file, not edit it in any way, but it may be sufficient, without the overhead of Pages or a similar app.