It's like anything else that's no longer supported. You continue to use it at your own risk. Sooner or later, you can expect it to fail, in one way or another (and I'm pretty sure I've seen one or two such issues on the recent past; I think in fact it was a display issue).
I don't know of an alternative. The script by Jack Yaz was VERY complex, and a work of art imo. The guy is absolutely amazing. I'm saddened we don't see him around here anymore.
Frankly, at this point, I think the better option is to use a router that support VLANs, like the more recent ASUS Pro offerings. Ironically, Jack Yaz eventually gave up on YazFi (and ASUS in general) due to the lack of native VLAN support (at that time).