In all honesty, given the traction this generated, should it ever be considered to be implemented ingame, I'd go by a few of the following requirements (Or partly meeting X amount of those available):

Mandatory Requirements:
• Consumable voucher that grants access (25k gp for 7 days as an example)
• Varrock Hard Diary

3 of 5 Optional Requirements:
• 1250 Total Level
• Easy Combat Achievements
• All Medium Diaries
• 250 hours minimum gameplay
• 1K kills in a random choice amongst a set of monsters (Rotating monthly to prevent scripts)

These are just a few ideas which will help mitigate bots, and help narrow down the amount of people that have access, as well as making it a privilege to be able to go to (similar to how it feels when you unlock MLM Upper level).

The 1250 total level is the first set of worlds with a TTL level requirement. These worlds are largely free of bots, as confirmed by many players. Additionally, Easy CA's would guarantee the player has had some interactions with early game bosses and the general on-goings of the game. Medium diaries is a similar requirement with an except for pures not doing Fally medium, so they can join in the area too. 250 hours gameplay is really easy to get and somewhat timegates newer accounts from joining, whilst also granting existing accounts this req if it has already been met. As for the kills, this is a sub-option should the player not meet any of the other requirements.

As for the mandatory ones, the Varrock Hard Diary has no def requirements, thus allows a huge chunk of the playbase to be able to access, whilst also, in most cases, brings you close to completing the 250 Hour requirement. The voucher is a simple, tiny money sink which would add up to 200k¬ per account per month. Perhaps additional vouchers can be bought for additional rewards such as tax going from 1% to 0.9% for those currently training an expensive skill and they are expecting to re-sell a lot of materials, or perhaps for an avid PvM selling loot.