Oh yes, encumbrance is quite the mess.
First thing to remember is that prior to 2e, encumbrance was measured in terms of coins, with coins being roughly equivalent to 1/10 of a pound. However, this wasn't just measuring weight, it was measuring bulk and the relative difficulty of carrying the item, so it was an abstraction.
However, the real issue behind encumbrance is that it has the players constantly recalculating things every time their PCs gain or lose an item, even a single coin. While it's necessary for balance in a system where the treasure is the biggest source of your XP, it's a pain in the ass to constantly be adjusting it on your character sheet.
0e and later Classic had flat rates for encumbrance, but Greyhawk first started adjusting encumbrance for Strength. 1e tweaked Greyhawk's numbers a bit I think, and it was based on categories of 35/70/105 lbs, with Strength adding a bonus or penalty. In typical fashion though, Gary wouldn't leave a simple system alone and so complicated matters by having heavier armor automatically applying a more severe category of encumbrance no matter how much was carried and by splitting the rules between the PHB and DMG again in different sections of the books while obfuscating things further by throwing in some vague exceptions. It's no wonder that between the constant recalculations and the scattered and confusing rules that encumbrance got often ignored in 1e.
Then 2e put its own mark on the system. It attempted to simplify things by dumping the concept of coins and converting everything to pounds. Coins was never the problem though, but at least 2e was more straightforward by putting all the rules together in one place in the PHB and explaining things clearly. But 2e did things in
its typical fashion by having multiple systems for the DM to choose from and then making the whole thing optional. It also had the most break points for encumbrance which made the system even more fiddly. The worst was the completely optional system where it broke down your movement rate by every step possible, so a human with a base move of 12 had break points for 11, 10, 9 and so on all the way down to 1. OTOH, 2e made XP for GP optional too, so the balance issues weren't as important.
3e further simplified things by going back to fewer categories, and later the game just switched the numbers to Strength x5/10/15 which eliminated another table. That's a good approach but, there's still the issue of constantly adjusting the numbers. So I've been looking into a slot-based system instead which seems to be a popular approach these days for the simplicity. While working on it, I looked into the history of encumbrance in the game, so I got to see just how complicated it got in the AD&D years.
This BTW ties into the discussion on spellbooks from about a week ago. As
@Mannahnin posted a few pages back, according to the UA rules, a standard spellbook weighs 150 gp and has an encumbrance of 450 gp (once again, Gary couldn’t make up his mind). Traveling spellbooks however weigh 30 gp with an encumbrance of 60 gp. In addition a backpack or large sack can hold a single standard spellbook, but a backpack can hold 5 traveling spellbooks and a sack 10 books. So here the advantage of the traveling spellbook is increased portability at the cost of worse durability and less space, but the idea seems to be that the wizard leaves the big heavy spellbook behind and uses the traveling spellbooks as a convenience. Of course, this detail is left out in the core 2e books because Zeb thought it more important to tell us there’s no such thing as a standard spellbook, that spellbooks vary not just between cultures but individual wizards, and then waxes poetic about exotic inks and aromatic paper. Interestingly enough, modern D&D has all spellbooks weigh the same as an UA traveling spellbook.