In order to remove the obsolete achievements without ruining for those who achieved them, you can award an artifact for each obsolete achievement a player has accomplished, and then remove the achievement entirely instead of keeping it hidden.
Convert Outdated Achievements to Artifacts
Are people that much of a completionist for that to bother them? Why should it be removed? I honestly feel happy that I'm one of the early gen knights since I've been playing since the release of this game. Groundbreaker Armor is also unobtainable now, do I care?
Well it's a bit more appropriate as an artifact than an achievement. Being listed as such implies it's a goal players should aim for. Also, having obsolete achievements listed can give the false impression that this game isn't worked on frequently enough or actively managed. Yes you're correct that it bothers people enough that they can't say they got all the achievements in this game. Outright removing them is a disservice to those like you who've weathered all or nearly all the changes this game has had. But, replacing the achievements for artifacts works for old and new players. You still have a record of your history, and newer players can feel a bit of pride of their own for completing every challenge set for them.
The artifacts could be medals designed with the image of the achievement and have a description to the effect of:
"Master Miner Medal"
"A medal awarded to the most experienced of knights"
"An award for accomplishing the difficult task of depositing 10,000 minerals into a gate."
That's why I made this thread yes.
"The artifacts could be medals designed with the image of the achievement and have a description to the effect of:"
Mater Miner => Win the 1987 Spiral Bowl.
=D