The references of a message were trimmed as per RFC 5537. Trimming means removing some references from ‘the middle’ of the references list to ensure, essentially, an unfolded length not exceeding 1000 characters. RFC 5537 pertains to Netnews articles and does not claim to cover email messages. RFC 5322, which describes the structure of email messages, does not mention any such trimming procedure. Its description of the procedure to create the references header content (Section 3.6.4) just says to prepend the msgid of the message being replied to to the references header content of that message to create the references header content of the message being composed.
So trimming is likely against at least the spirit of RFC 5322. Moreover, given that current msgids can be of the order of 70 characters, trimming would happen already around the 15th reply, which is not an unrealistic thread length. This means that thread reconstruction can be made difficult in case a user does not keep (or even received) every message in the thread.
This change removes the references trimming step.