WORKAROUND:
You can revert to the behavior seen in 13.2 by following these steps:
1) Shut down Toad
2) Browse to C:\Users\username\AppData\Roaming\Quest Software\Toad for Oracle\17.0\User Files folder in Windows Explorer
3) Find the File called ProjectConfig.txt and double-click to edit it with Notepad
4) Scroll down to the [ASSOCIATIONS] section and you'll find the line for SQL*Plus. At the end of the line you'll see 'TRUE'. Change it to FALSE.
5) Save and Exit.
Now that extension should work as before.
STATUS:
Waiting for fix in a future release of Toad for Oracle