In order to get the shave the smallest of timesaves, clever tricks are used. However as Minecraft itself updates, it often removes these trick, where once mcci updates to require the newer version of MC, matching the old run(s) is no longer possible. Here are some examples already.

Some changes would impact all runs for a given map

Patched in 1.20 Wouldn't get a block effect on the edge https://bugs.mojang.com/browse/MC-1133

This really really only impacts beehive as WR uses it to bypass honey slowdown at start. WR video is now missing from that did that... (could've been used on air train with end slime but don't think anyone did)

Patched in 1.21.2 https://bugs.mojang.com/browse/MC-275834 (see bottom comment) Pressing space on slime blocks no longer cancels your jump. This is actually a 1-2 second time loss on the ending of air train with the slime blocks, as holding space lowered your bounce while keeping your horizonal speed. Basically all runs on the leaderboard used this

Patched in 1.21.4 (releasing tomorrow) Sprint Sneaking: https://bugs.mojang.com/browse/MC-152728 this is about how you can move faster by holding your sprint key while moving before you press shift.

This impacts any run/map that uses sneaking, with Chilli Con Carnage being hugely impacted as it used for all the slab sections with ice. Funhouse is also slightly impacted.

Worth mentioning here, this also massively impacts parkour warriors as well since it is often used in ice parkour on top of shifting under slabs (hopefully it doesn't make one flat out impossible)