It's probably not the absolute cap as I've likely missed some effects, but in the character planner I can get to 98.9% cold absorption with this breakdown.
EDIT: Unfortunately I just realised that you can't make a LGS weapon sentient, so you'd have to drop the shield and wear LGS in your off-hand which makes the total 98.7%
I'm also not entirely sure that the LGS bonuses are indeed Equipment and Exceptional for tier 2 and 3 respectively. That's purely based on the planner (the wiki doesn't specify their types).
If they are regular Enhancement bonuses then the total is reduced 97.6% (at least you can now wear the shield again).
3 - Past Life: Arcane Sphere: Ancient Knowledge
3 - Past Life: Arcane Sphere: Arcane Alacrity
3 - Past Life: Arcane Sphere: Eclipse Power
3 - Past Life: Arcane Sphere: Enchant Weapon
3 - Past Life: Arcane Sphere: Energy Criticals
30 - Past Life: Divine Sphere: Block Energy
15 - Pale Master: Undead Augmentation I: Undead Chill
15 - Unyielding Sentinel: Mantle of the Sentinel
25 - Grandmaster of Flowers: Flow of Water
25 - The Flow of Water + Ocean Stance
10 - Favored Soul / White Dragon Dragonlord / Cold Warlock
15 - Primal Avatar: Weathering the Elements
15 - Shrine to the Devourer IV
50 - Fire Shield: Fire
10 - Legendary Guardian of the Gates: Energy Absorption
1 - Cannith Crafted Cloak : Deck Curse : Curse of Divine Fortune
38 - Cannith Crafted Trinket : Cannith Trinket Prefix : Cold Absorption
2 - Curse of Major Masterwork (On Cannith Trinket)
30 - Greensteel Weapon : Greensteel Weapon Tier 2 : +30 Equipment bonus to Cold Absorption
35 - Greensteel Weapon : Greensteel Weapon Tier 3 : +35 Exceptional bonus to Cold Absorption
15 - Legendary Mirrorplate Tower
5 - Filigree 1 (Weapon): Frozen Wanderer: Cold Absorption
5 - Filigree 2 (Weapon): Dragonsoul: Cold Absorption
5 - Filigree 3 (Weapon): Snowpeak's Gifts: +5% Cold Absorption
5 - Filigree 4 (Weapon): Nystul's Mystical Defence: Cold Absorption
5 - Filigree 1 (Artifact): Frozen Wanderer: Cold Absorption
5 - Filigree 2 (Artifact): Nystul's Mystical Defence: Cold Absorption
5 - Filigree 3 (Artifact): Dragonsoul: Cold Absorption
5 - Filigree 4 (Artifact): Snowpeak's Gifts: +5% Cold Absorption
It would be a complete gimp build, but we're just trying to see what is possible
You could also cast Greater Inflame on yourself (if going the FvS route - if not just get a friend to do it) for another 15% for 30 seconds every 1 minute.
And if you had a level 18 dragonlord friend with a white dragon soul then you could get another 5% from his aura.
If what Lominal says is true (that rng loot can have 40%) then you can add another 2% as I'm basing my number on the max possible through cannith crafting which is 38%.
There's an enhancement for Winter Wolf that gives 30% cold absorption, however it doesn't list its type and the planner puts it as Enchantment so it doesn't stack with Fire Shield. If it's a stacking type that would be another 15% over palemaster (can't have both).
You could also go Water Elemental instead of Palemaster for the same result.
It's also possible that the heroic greensteel absorption effects stack which would potentially mean that it's better to wear a HGS with a LGS weapon. This would forego the weapon filigrees, but would net you 25% extra absorption. Legendary and heroic likely share the bonus types, though, so this is just a wild theory. If only tooltips actually showed the bonus types, right?
It might also be that the heroic greensteel types aren't Enhancement, but the legendary ones are. In that case you'd wear the HGS in the off-hand and still drop the shield. The numbers would be a bit lower than with LGS, though.
I'm not sure exactly what that would put you at as the math is a bit complicated and I'm relying on the builder to do it for me
Also there's the feat "Shield Deflection" which would give you a 40% chance to completely negate any elemental damage taken. That's slightly different, but worth noting.
Fire is probably the absorption that you can get the highest (other than 100% ones like negative/poison from immunities).
This is because they have better items (you can get 10% extra from filigrees) and there's a runearm with 6% quality resistance (not relevant if LGS doesn't stack as you'll be wearing the shield). Also if the screenshot of Epic Purging the Pantheon is to be believed then it grants 43% fire absorption compared to the 38% possible for other elements.
You can also get an extra 5% from Exalted Angel.
Electric can go slightly higher than cold due to Tempest giving 30% from it's imbue. You lose 10% from filigrees, though, and you can't get the 10% from FvS/Dragon Lord).
Acid is likely the worst of the elements. You lose 20% from filigrees compared to cold and there's no elemental form or palemaster enhancements. There is, however, an enhancement for blightcasters to give you 30% acid absorption in wolf form. This would make it better, but it might suffer from the same non-stacking as the winter wolf one.
