Before the era of development on GitHub, it likely would have been in past era where there would be a Wip announcement on one of the devs websites or Twitter account. Those days are somewhat over now that visitors can look at progress on some GitHub accounts these days.
Pull requests are created in hopes to be added to source code in the future. Each request is viewed and corrected over time in case devs believe improvements are needed first in order for code to be supported.
When at front page of MAME Git Hub account, the miniature icon over on far right side can be clicked on to see what pull requests have been officially accepted into MAME source code.
>It appears marble madness 2 will be in the next release of mame...
It will be in following .245 official release, but since it is now currently in the source code, it is possible to compile an unofficial build……that is for those users that have modern era computers and know how to work on creating their own builds while most users will wait until official release is out.