maushold didn't exist in gens 8 and 5.
and cloyster was still available at gens 6 and 7 but king's rock wasn't banned there. why wasn't it banned there?
(also I don't think that cloyster abused this item thaaaaaaat much, not compared to dpp jirachi's paraflinch which had 70% chance of success each turn compared to cloyster's 41%. dpp jirachi was very ban worthy.)
There is not always pressure to look at RNG items (King's Rock, Bright Powder, etc.) because their unreliability tends to result in low usage. However, it's implicitly understood that if usage ever picks up (which hurts the competitiveness of the game), they can be banned at will by the council because do not have a legitimate competitive usage. Removing it does not change the metagame beyond removing an unreliable cheese strategy. The only reason it's not banned in Gen 6 and 7 is because nobody spammed King's Rock Cloyster enough to cause an outcry from the userbase.
The situation of Jirachi is incomparable because it is heavily bound into the metagame Gen 4 OU. It cannot simply be removed because it may heavily disrupt an old metagame, so there has to be agreement from the competitive playerbase to take action. From a practical standpoint, it is also easier to find checks which can take an unboosted Iron Head than it is to find checks which can repeatedly take +2 Icicle Spears and Rock Blasts.