Roblox was tiny in 2006. I joined in 2008. It was still leading the market.

To give an example, Roblox added user-created cosmetic t-shirts as a way to monetize the platform. Developers immediately scripted their games to recognize special "VIP t-shirts" that would provide in-game benefits. And quickly created idle games called "tycoons" where you could wait 2 hours to accumulate money to buy a fortress, or buy the t-shirt to skip all that.

I don't think there were any modding systems with mtx support.