What do you mean? Autocommit mode is the default mode in Postgres and MS SQL Server as well. This is by no means a MySQL-specific behavior!
When you're in autocommit mode, BEGIN starts an explicit transaction, but after that transaction (either COMMIT or ROLLBACK), you return to autocommit mode.
The situation being described upthread is a case where a transaction was started, and then rolled back by the server due to deadlock error. So it's totally normal that you're back in autocommit mode after the rollback. Most DBMS handle this identically.
The bug described was entirely in the client library failing to surface the deadlock error. There's simply no autocommit-related bug as it was described.
Lack of autocommit would be bad for performance at scale, since it would add latency to every single query. And the MVCC implications are non-trivial, especially for interactive queries (human taking their time typing) while using REPEATABLE READ isolation or stronger... every interactive query would effectively disrupt purge/vacuum until the user commits. And as the sibling comment noted, that would be quite harmful if the user completely forgets to commit, which is common.
In any case, that's a subjective opinion on database design, not a bug. Anyway it's fairly tangential to the client library bug described up-thread.
Autocommit mode is pretty handy for ad-hoc queries at least. You wouldn't want to have to remember to close the transaction since keeping a transaction open is often really bad for the DB
