Unlike brass instruments, woodwind instruments have finite range – at least on the bottom of the instrument. For the Bassoon, this is the bottom B-flat below the bass clef. However, this has not stopped composers from ignoring this rule. Low As are seen with enough regularity that bassoonists almost don’t even bat an eye at them. They occur in nearly every work by Mahler, and in the latter operas of Wager as well as scores of other works. However, an extension on the bassoon to reach this note is rare. Heckel offered it as an option for years, but seems to have dropped it from their catalog. When it was offered, it was done as a second bell. The traditional B-flat bell was used for the majority of the time, and the low-A bell was only brought out when needed. In my 20 some-odd years of bassoon playing, I have only ever seen one true low-A bell. However, the low-As still exist. The easiest way to produce the note is to stuck a tube in the bell. I have a plastic tube that projects about 7 inches above the bell and produces a fairly nice low-A. However, the low B-flat is completely unplayable. When the extension is in, many other notes are affected, so it’s best to have the extension in for as short of a time as possible.
Conclusion: low-A is fairly normal, but give the player time to insert the extension, and don’t write a B-flat and an A together.
Contra is a totally different story. Contras with low As are fairly frequent.maybe up to 10% of instruments have the note. However, the note is almost never scored for. The only example I know of is in Elektra. In my own Contrabassoon Concerto, I heavily use the low A, but I score this work with a Contraforte in mind which always descends to that note.
Here is a video showing extensions for both instruments.