MAMEUI64 0.289.5 has been released.
Available at https://messui.1emulation.com
There's been a lot of changes, mostly to do with avoiding the possibility of buffer overruns. More to do though.
Why release now? There hasn't been a release since July, and there's no official explanation why the delay, or when 0.290 might happen. A lot of games have been improved, so you might as well get the chance to try them out.