I suspect that a majority of players don't give a hoot about Achievements, and Challenger Achievements less so, but I do! And I especially enjoy looking for all of the Challenger Achievements. So it irks me that there is a completion bar for Challenger Achievements when a good portion of them aren't even attainable anymore (all of the July achievements from the Apedemak/Menhit/Apollyon storylines), as that means this bar will never reach 100% for people that didn't play for at least 5 years prior to the event being retired (at least 2015, since the last Falling Stars event was July 2020 before being swapped with Meteor Showers in July 2021 and there are 5 endings/5 achievements (wiki)).

I would suggest creating a new Achievement category purely for unobtainable/retired/limited time achievements under the name Legacy Achievements, or something to that effect? Or Legacy Achievements strictly for retired events and something like the "Feats of Strength" achievement category from World of Warcraft for limited time/nigh-impossible achievements that don't count towards any completion progress? Just something that separates them from the Challenger Achievements so that they are still challenging but, you know, achievable.

EDIT 7/1/2023: It appears that they've reinstated the Civil War storyline with this year's July event, though I'd like to keep this suggestion up for further cases as it may apply in the future as well!