The older Sentinel data are available upon request in the Long Term Archive (LTA) Access. For the current status, see https://scihub.copernicus.eu/userguide/LongTermArchive .
You should be able to order data from the LTA either through the Copernicus Open Access Hub (https://scihub.copernicus.eu/dhus/#/home) which requires a registration.
An alternative in Python is to use sentinelsat which also offers LTA support:
https://sentinelsat.readthedocs.io/en/stable/api.html?highlight=lta#lta-products
Important: here I just read that you need to install sentinelsat from its master branch at https://github.com/sentinelsat/sentinelsat as it hasn't been released yet (ref). See also a related closed issue on the matter.
All a bit tricky!