Uploading acepted ACL paper to arXiv I have written an accepted paper for the 2020 Annual Conference of the Association for Computational Linguistics (ACL). While uploading the final version, I agreed to this copyright statement:


  Copyright to the above work (including, without limitation, the right
  to publish the work in whole or in part in any and all forms and
  media, now or hereafter known) is hereby transferred to the
  Association for Computational Linguistics (ACL), effective as of the
  date of this agreement, on the understanding that the work has been
  accepted for presentation at a meeting sponsored by the ACL and for
  publication in the proceedings of that meeting. However, each of the
  authors and the employers for whom the work was performed reserve all
  other rights, specifically including the following: (1) All
  proprietary rights other than copyright and publication rights
  transferred to ACL; (2) The right to publish in a journal or
  collection or to be used in future works of the author's own (such as
  articles or books) all or part of this work, provided that
  acknowledgment is given to the ACL and a full citation to its
  publication in the particular proceedings is included; (3) The right
  to make oral presentation of the material in any forum; (4) The right
  to make copies of the work for internal distribution within the
  author's organization and for external distribution as a preprint,
  reprint, technical report, or related class of document.


I would like to upload the same final paper to arXiv. Am I allowed to do so? 

Specifically, I think exception (4) might be relevant here, as the proceedings aren't published yet, so I guess it would be a pre-print? However, it is the same final paper, not some intermediate result.

I am aware that uploading the accepted papers to arXiv is common practice. Many authors advertise this eg. on Twitter, so ACL must be aware of it. But do they just ignore it, or is it actually legal?
While only they can say for sure, it is probably fine to do so, but you also probably need to include their copyright notice with the uploaded version. 

But to be certain, ask them specifically for guidance. 

It isn't a "preprint" as you note, but it is a "reprint" and probably falls under the "related" class of documents.