Penultimate isn't that uncommon? Just because you don't personally hear it in your day-to-day life doesn't mean other people don't use it.
https://books.google.com/ngrams/graph?content=penultimate%2Csecond+last%2Csecond+to+last%2Cnext+to+last&year_start=1800&year_end=2022&corpus=en&smoothing=3
Penultimate isn't that uncommon? Just because you don't personally hear it in your day-to-day life doesn't mean other people don't use it.
https://books.google.com/ngrams/graph?content=penultimate%2Csecond+last%2Csecond+to+last%2Cnext+to+last&year_start=1800&year_end=2022&corpus=en&smoothing=3