[2021] Specific mod drops and how they're tied to zone level

This is just RNG, and RNG often feels less random than it actually is, because unlikely events can randomly clump together and make it look like there's a pattern. If you roll a dice three times, it's unlikely that it'll roll three sixes, but if you do the same with hundreds of dice, some of them probably will. With the shear number of different mods in the game you're going to end up with a few where most or all the drops end up randomly clumped.

Affixes have a fixed weighting once you reach the level at which they drop, so they only ever become less common when more mods start dropping and the pool is diluted. The weighting never goes down.

It applies to all loot, except for boss specific unique drops.

Is there a chance as the zone levels progress the weight stays the same for these mods (as you say), but the weight for the other newly unlocked mods is slightly higher?

Most mods available early in the game have the maximum weighting and mods that start dropping later normally have a lower weighting as they tend to be more niche or powerful.

To explain the statistic, imagine a simplified example where there are four possible mods, A, B, C, and D, and each have equal weighting. In that scenario you have a 25% chance to get mod A. If at a higher level there are six more possible mods and all 10 have the same weighting you only have a 10% chance to get mod A.

the mod of what I'm looking for back at lvl 75 or so is essentially lowered because of its relative weight to the newer mods

There are no affixes will level requirements above 60, so the affix weightings at level 75 are identical to the affix weightings at any other level from 60 to 100. Higher levels at that point just mean the affixes will roll at higher tiers more often.