The osCommerce project is not dead, but it is primarily in the hands of the community at this point; the leadership team seems to be inactive.
The last release of the product was done by the community and is called "Frozen." It can be found here: https://forums.oscommerce.com/topic/412924-frozen/.
Frozen is based on Bootstrap (BS) and will work under PHP 7.1+.
The Frozen release was the basis for a Github project which is currently under development by osCommerce legend Gary Burton. This is referred to as the Community Edition (CE), and is available here: https://github.com/gburton/Responsive-osCommerce.