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 most recent iteration of osCommerce is the Phoenix release, which is currently under development by osCommerce legend Gary Burton. This is sometimes referred to as the Community Edition (CE), and is available here: https://github.com/gburton/Responsive-osCommerce. Phoenix is mobile friendly and works under PHP 7+.
Prior to Phoenix, 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+.