In Adventurer's League Season 7, which creatures are PCs allowed to polymorph themselves or others into? Do they have access to the entire SRD, or only a limited set of creatures?
I don't think there is any particular restriction for AL. The normal restrictions for polymorph apply:
The new form can be a beast whose challenge rating is equal to or less than the target's CR or level.
There is a ruling in the AL FAQ, it doesn't list polymorph by name but should apply just the same:
The Monster Manual and other resources are legal resources for the purpose of determining what is available for class features such as Wild Shape, Beast Companion, and various conjure spells, noting that, as always, your DM is the final arbiter for the rules—including determining whether or not your character has satisfied requirements such as those imposed by your class (such as the druid’s requirement to have seen the beast in question). Additionally, without specific campaign documentation, you can’t use any of the Variant rules contained therein. Finally, as there are no specific stat blocks for “templated” creatures, such as half-dragons, these are not permitted for use—only creatures with an official stat block are available to players and DMs.*
From a practical stand point many DMs would dis-allow or question how you could select a creature that doesn't exist in your campaign world or your character has never heard of, say Dinosaurs before you get to Chult in Tomb of Annihilation.