Sorry to reply to my own post, but I had left a wrong (or at least incomplete) impression that I want to correct. While it is true that a lot of the symbols have disappeared from the long-press '.' key, I have found out that there is a compensating workaround: when you press the "?123" key, the keyboard immediately switches to the numeric/symbols keypad. If, instead of releasing the "?123" key, you swipe to a symbol, that symbol will be inserted and the keyboard will immediately revert to the alphabetic keypad.
For me, at least, that's just about a wash with the old long-press '.' method: there's a longer swipe to the parentheses, for instance, but there's no long-press delay, either. That has removed my disincentive for upgrading to the current Google keyboard, so I wanted to pass that along for anyone who had the same reaction to the missing symbols that I did.