From @coke

19​:04 < [Coke]> r​: say "ü" ~~ /​:ignoremark 'u'/
19​:04 <+p6eval> rakudo 13e805​: OUTPUT«===SORRY!===␤Unrecognized regex
modifier
  :ignoremark␤at /tmp/93aS3IV3lu​:1␤------> say "ü" ~~
  /​:ignoremark⏏ 'u'/␤»

From the spec​:

The :m (or :ignoremark) modifier scopes exactly like :ignorecase except
that it ignores marks (accents and such) instead of case. It is equivalent
to taking each grapheme (in both target and pattern), converting both to
NFD (maximally decomposed) and then comparing the two base characters
(Unicode non-mark characters) while ignoring any trailing mark characters.
The mark characters are ignored only for the purpose of determining the
truth of the assertion; the actual text matched includes all ignored
characters, including any that follow the final base character.

--
Will "Coke" Coleda

From @FROGGS

Patch​: rakudo/rakudo@4cfec0c38d
Test​: Raku/roast@6ecf415d8a

@FROGGS - Status changed from 'new' to 'resolved'