It has returned to normal behaviour, i.e. defaulting to main but configurable (via both command-line option and config file) to use test.
I mused on it for a bit, trying to get a better understanding of the purpose of testnet.
Bitcoin Core now has three networks, main, test and regtest.
The way testnet coins are now generated points to a difference - for Bitcoin Core, testnet coins are generated via RPC console command generate
In principle, the public ledger must be safe from J. Random User's dedicated-but-individual attempts hack their copy of the client to suborn the network to their personal advantage. I am no different from J. Random User except that my intentions are allegedly beneficial (says me) so by the same token, the network should be safe from any of my individual efforts whether deliberately or inadvertently mounted. Of course, if I can persuade > 50% of users to join my camp, then whatever I've done to the protocol/functioning will become the new norm. Testnet gives me some protection from any inadvertent ganging up on the currently-accepted consensus but then again, I hadn't introduced any significant changes to the functioning of the engine, merely adjusted the timing belt, to use a motoring analogy.
But I'm always acutely aware that the numbers represent far more than just public ledger accounting - humans are involved.
Cheers
Graham