One pertinent section entitled "multiplatform services" says "[a]pps that operate across multiple platforms may allow users to access content, subscriptions, or features they have acquired elsewhere, including consumable items in multi-platform games, provided those items are also available as in-app purchases within the app." It also says developers must not discourage the use of in-pp purchases, even though they offer their goods elsewhere.
But it's section 4.2.7 of the revised guidelines that talk about mirroring apps themselves. It says those apps must be managed from a PC, and users must only be able to create an account on the host device. More importantly, they must "not provide a store-like interface, or include the ability to browse, select, or purchase software not already owned or licensed by the user." Since Steam Link is a multiplatform app, though, it's allowed to offer purchases outside of Apple's control, so long as they're made on the host PC. The company previously said that it's working with Valve to fix the situation, so expect the application to be compliant with those guidelines.
In addition to new rules for mirroring apps, Apple's revised guidelines include a section that says non-subscription apps can now offer time-based trials "before presenting a full unlock option." The company also made it clear that apps must not mine cryptocurrency in the background -- it once approved a Mac app with a misbehaving crypto-mining feature -- and told developers that their applications must have "appropriate security measures to ensure proper handling" of any user info they collect.