This is both reasonable and a common GNUism that a lot of ported software expects.
Universally process \r, \n, and \t into carriage return, newline, and tab respectively. Newline still doesn't function in contexts where it can't (e.g. BRE), but we process it anyways rather than passing UB \n (escaped ordinary) through to the underlying regex engine.
Adding a --posix flag to disable these was considered, but sed.1 already declares this version of sed a super-set of POSIX specification and this behavior is the most likely expected when one attempts to use one of these escape sequences in pattern space.
This differs from pre-rS197362 behavior in that we now honor the three arguably most common escape sequences used with sed(1) and we do so out side of character classes, too.
PR: 229925