Technically, at a protocol level, there isn’t a “default” for Z or T address use at all, they are treated equally.
The only “default” action that is required is all coinbase transactions must first be sent to a Z-address before it can be spent, so in that regard Zcash is “private by default”. However, once that has happened, then it is entirely up the users who interact with the Zcash choose whatever addresses they use. First steps often being for the pool to distribute ZEC to miners, and miners to then to do whatever miners do with thier Zcash.
The friction to using Z-addresses used to be CPU/RAM overhead. Now that that has been solved the friction has become the tooling/code for smartphones/apps, engineering overhead (initial and ongoing) for a major player like an exchange to switch, and possible hesitation by exchanges due to regulatory concerns.
That said, any exchange which currently supports any coin with privacy features (regardless of the strength of that feature) like Monero, Dash, Verge, etc… (IMHO) should support Z-addresses.