| Taproot Leaf Script
| <tt>PSBT_IN_TAP_LEAF_SCRIPT = 0x15</tt>
| <tt><control block></tt>
| The control block for this leaf as specified in BIP 341. The control
block contains the merkle tree path to this leaf.
| <tt><script> <8-bit uint></tt>
| The script for this leaf as would be provided in the witness stack
followed by the single byte leaf version.
So far, all the defined PSBT types had a relatively short keydata (not much bigger than a couple of pubkeys).
I think that is a desirable property to keep, as it is often a reasonable assumption that dictionary keys are not very large.
The control block as per BIP 341 can be up to 33 + 32*128 = 4129 bytes long.
Perhaps it would be better to split this into PSBT_IN_TAP_LEAF_SCRIPT and PSBT_IN_TAP_LEAF_CONTROL_BLOCK (both with no keydata)?
Best,
Salvatore Ingala