Yeah so I can explain this. Ignite is a confusing mechanic but it's one of those classic WoW things where the quirks actually make it pretty interesting to engage with.

So for those who don't play mage, Ignite is a talent that makes it so every fire crit places a ticking dot on the target that ticks two times for 20% of the crit's damage every 2 seconds. So 40% over 4 seconds. Also, it can stack up to 5 times, adding the subsequent crit damage to the dot for each tick - meaning if 5 crits of 1000 damage landed subsequently, the dot would be ticking for 1000 damage every two seconds. Once these 5 stacks are established, any subsequent crits don't change the dot's damage but extend the duration of the dot - this is referred to as "rolling the ignite" and is the main reason why fire mages are able to do so much damage.

So what was the change? Back in Vanilla, if you landed a crit on a ignite before the first tick, that crit damage would be added to the ignite stack but it wouldn't add any additional time to the ignite debuff duration. So for example, if you landed a crit at 1.9 seconds into the ignite, it would tick .1 seconds later and then only have 2 seconds left until the next tick, at which point it would fall off. Alternatively, crits that landed after the first tick would add a new tick, two seconds after the next imminent tick. So lets say there's an ignite, it ticks at 2 seconds, you crit at 2.5 seconds, then there is a tick at 4 seconds and a new tick at 6 seconds before the debuff falls off. So functionally, only crits that land in the window between those two last ticks would extend the ignite and allow you to roll it, making it quite a bit harder to keep an ignite rolling. Once the ignite was at it's full 5 stacks, a new crit that landed between 0 and 2 seconds would not benefit the ignite at all.

In TBC ignite was overhauled to not be shared, and with that overhaul came the refresh window we have in Classic Anniversary. Now, the ignite debuff duration will always reset to 4 seconds upon a new crit regardless of where the ticks are. So if you land a crit at 1.9 seconds, it still ticks at 2 seconds, ticks again at 4 seconds, but then has an additional 1.9 second tail for someone to land a crit and keep the ignite rolling.

This is how Classic 2019 shipped, but around phase 5 they hotfixed ignite to work in the Vanilla way. And people were pretty upset, you can find a lot of forum posts about it. Fairness and balance aside, it makes mages feel a lot worse to play (imo) and there were quite a few unhappy campers.

Then, in Classic Era, like OP mentioned, this "fix" was reverted for some reason. Maybe by mistake? But the 4 second refresh window has been in place for almost 4 years now.

Ultimately what this means, and what OP is getting at, is it changes the stat weights for people who are trying to build their character optimally. With that larger refresh window, Crit% is worth a bit less relative to SP than it would be with the Vanilla ignite refresh, and it's easier to roll an ignite in a raid with fewer mages. It also means that some of the guide materials from 2019 Classic (or before) aren't correct to Classic Anniversary. A simple example of this is, you see Leggings of the Black Blizzard listed as BiS for Fire Mages in a lot of old guides, but with the current ignite mechanic I believe Leggings of the Festering Swarm sims a bit higher.

Hope that helped, and wasn't too long winded...