The last stable #OpenBTS is more than 10 years old now
I don't know if the project is dead or not, but it was handy to setup a GSM base station to make your own DIY cellular network, especially with older mobile hardware
I guess WiFi has taken over this space and maybe it's not seen as being valuable, but I think it's good to still be able to use your pre-4G mobiles. Even if it's on a private network
There are lots of rural areas where cellular access isn't viable and yet, the lower frequencies (< 2.4 GHz for WiFi) get through trees and other obstacles more easily
The Open Source Mobile Communications page has listings of other projects
https://osmocom.org
One of the few that seem promising to take over for OpenBTS seems to be DragonOS. There's a YouTube channel:
https://www.youtube.com/@cemaxecuter7783
As an aside, 900 MHz is in the ISM band for ITU Region 2 (mostly, the Americas). That's well within cellular territory for a lot of GSM mobiles