In short, item staking has been removed, and only GP and platinum tokens can be used for stakes.
Everything You Need To Know About The RS Duel Arena Update

Duel Arena Update – Tech Aspects
Avoiding The System
Final Thoughts About This Duel Arena Update

This was done to ‘tackle odds staking and Real World Trading’ while, in reality, it just causes harm to the community. In general, this move creates more problems than benefits for the community. Let us explain!
RS Duel Arena Update – Tech Aspects

There are a few rules added to the ‘Duel Arena staking’ after the update. These are:

Staking tax – your stakes will now be subject to tax. A certain percentage will be deducted from your total stake. Much like how the government taxes you in real-life, huh?

o 0.25% - if the stake is anywhere between 0 and 10 million GP

o 0.50% - if the stake is between 10 and 100 million GP

o 1.00% - if the stake is higher than a 100 million GP
This means that if you decide to duel and stake 101M GP each, you would now claim less than 200M after the victory. While before the update, you would get 202 mil. That is more than a two million GP difference, which could amount to a lot for those who often wager higher stakes.
The strangest question is: ‘Where do the taxes go?’. No one knows… In reality, we believe that this money disappears.

Item staking is removed – no duels in the Duel Arena will allow item staking. This is done to fight odds staking and RWT. However, it just causes harm to honest players who stake because you will not be able to fight for something valuable, having to use cash or platinum tokens instead. Ironmen, however, cannot stake anything. Jagex probably thinks that players will move to PvP worlds and the Wilderness instead. We do not think so.

In reality, this just crashed the economy of expensive items. Previously, OSRS items like Elysian Spirit Shield or the Twisted Bow along with other rare and exclusive items were costly, so they were incredibly lucrative for players to stake and try winning. Now, the prices drop significantly, meaning that if someone won it recently and was looking to sell it for max profit – it’s too late.

For long-time owners, this is not so cool as well. They will lose a significant chunk of their bank because quite a lot of these items lose their value rapidly.

On the other hand, it can be beneficial for people buying OSRS Gold at Probemas. It is the perfect time to get your gold with a discount code and purchase the item of your dreams. Justicial Armour or the same Twisted Bow can be yours at ridiculously low prices.
Staking difference – your stakes must now be within 10,000 coins in value. So you cannot stake 200M with the other player staking nothing. He or she has to stake 200M +/- 10k coins.

Avoiding The System

Jagex removed a very integral part of the Duel Arena – item staking. They also added taxes that removes gold from the game. But probably the worst part about it is the taxation because it creates a massive hole for betting and gambling sites. They can offer a lower tax and receive a lot more customers than they had had before.

For example, the odds staking prevention does not work. People offer to duel with Rune Boots vs no Rune Boots or while the other player wears a ring of recoil etc. These cunning findings by fellow players allow the community to work around these restrictions.
Final Thoughts About This Duel Arena Update

While this prevents some illegal activities in the world of Runescape, people have already found ways to avoid the system. Either through outside sources like other sites or dropping the items, later on, it only seems that the RuneScape community is going to find new ways to work around this in the future.

In our eyes, Jagex tried to fix something that was not broken in the first place. While some essential features in the game lack innovation, RS mobile is not yet available to all mobile platforms and other crucial details falling short of our expectations, we think that this update was not necessary. And anyway, people have already found ways to outwork it.