Looking at the life cycle of MacOS through the Wikipedia page, it seems like Yosemite had a run of about 2-3 years before being no longer supported by Apple. Roblox is dropping support for Yosemite later this year, making its run about 5 years on Roblox’s end.
Judging from that, I’d say you probably have about 3 years (if not more) before Roblox drops support for Mojave, assuming they continue the cycle of updating their engine and their minimum OS requirements. Hopefully by that time (~3 new releases of MacOS) you’ll find a more updated version of MacOS that fits you better.