How the transaction logs are maintained in MSSQL RDS?
RDS is closed source and hidden, but assuming that it is like PostgreSQL in that respect:

The PostgreSQL transaction log is managed differently. It is split into segments of 16MB (by default). As soon as the current segment is full, the next one is used. The full transaction log segment is archived and deleted. These archived logs are not managed by PostgreSQL, and you keep them around as long as you want to be able to restore the database.

So it makes no sense to say that the transaction log gets full or that it gets shrunk, all this is not necessary.