Bitcoin Script is Forth-like RPN, why does it need AST? Do you mean there are plans to add more languages/DSLs to Bitcoin which with multiple compiler front-ends generating ASTs. which then translated to "machine opcodes"? Something similar to Ethereum?

Or maybe it's a solution for transaction scriptSig malleability? [1]

[1] https://en.bitcoin.it/wiki/Transaction_Malleability