The spec provides a PUT method for setting account data, but the only way to read the data back is from the sync queries.
This is very nice if you want to be able to get a stream of data between clients, but in the case of bots that just use the account data to store configuration / state between restarts - or use the HS as a key-value store - then it's more of a nuisance.

Adding GET methods that will directly retrieve the latest data in the given id would be helpful for those use cases;

GET /_matrix/client/r0/user/%40alice%3Aexample.com/account_data/org.example.custom.config
Content-Type: application/json

{
  "custom_account_data_key": "custom_config_value"
}

GET /_matrix/client/r0/user/%40alice%3Aexample.com/rooms/%21726s6s6q%3Aexample.com/account_data/org.example.custom.room.config
Content-Type: application/json

{
  "custom_account_data_key": "custom_account_data_value"
}

A Last-Modified header could be nice to have, but most definitely not a necessity for the feature.