I think any substantive change to the regulation of games of chance inside games will focus on two aspects, (1) real world currency exchange for direct entry in to the game and (2) the effective rate of access to the game of chance.
In the first instance any new law (or incorporation in to existing law) will be primarily aimed at stopping vulnerable people from paying excessive amounts of real money for access to the gambling option. I don't believe this affects OSRS, RS3's Squeal of Fortune might be dead as a dodo though.
In the second instance I don't believe clue scroll reward caskets would fall afoul of any legislation relevant to the rate of access of the game of chance. Gambling, in "look and feel" (from the House of Lords' report) does not include the completion of a 20 minute minigame in order to reach the game of chance element of the minigame to any reasonable person. It also does not include the inability to access the minigame at will - gambling in "look and feel" must include the ability to enter the game of chance upon choosing to do so. Receiving a clue scroll drop is random. Entry in to the game of chance therefore is not controlled by the player and is effectively invite only.
As for the game of chance itself, the reward casket, as no entry fee is required and the access to the casket not immediate I do not believe it would be regarded as gambling by any reasonable person which is usually the basis of UK law, and if it did fall afoul of any new law it would either be an excessively undefined law or an unfortunate happenstance which can simply be avoided by generating the reward upon receipt of the clue scroll and listing the reward before the player completes any task. This might sound silly, but since a player can only have once of each difficulty scroll at a time the player must grind through the obby cape rewards before he or she can get to the 3a rewards. The process is no different, there's just no sense of excitement while grinding a cape.
02-Jul-2020 18:26:26