"The biggest question for me is will this cause a need to replace this as well?"

By itself, no. A number of things kill an O2 sensor but lack of back-pressure is not one of them.

"if it was in good condition prior to this, will it still be useable once the new exhaust system is in place?"

Maybe...but did you test it prior? How long has it been in this condition? It the sensor portion exposed to road salt, oil and grime?

Bo not be penny wise and pound foolish. If you do not know if it is function and you do not want to get under the car in the near future, just have it replaced when the exhaust is replaced.