Normally I'd say a footnote is overkill, but since there is that note

Note that any escape sequence using braces inside interpolated constructs may have optional blanks (tab or space characters) adjoining with and inside of the braces, as illustrated above by the second \x{ } example.

every "escape sequence using braces" should better have this as footnote plus "(since v5.34.0)"

On a side note, this seems to be in analogy how identifiers inside braces are handled

  DB<70> $a = 42
                                                                      
+                                                                     
+                                                                     
+                  
  DB<71> p ${ a }                                                     
+                                                  
42                                                                    
+                                                  
  DB<72> p ${  a  }                                                   
+                                                  
42                                                                    
+                                                    
  DB<73> p "${  a  }"                                                 
+                                                  
42                                                                    
+                                                    
  DB<74>
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