Right around a month ago, Intel announced it had completed the release of microcode updates for products launched within the last 9+ years to protect against Spectre and Meltdown. It was then up to Intel's hardware partners to issue BIOS updates based on the latest microcode, and Asus has begun doing that for some of its older products. Specifically, Asus now has new firmware available for several boards based on Intel's 9-series chipsets.
Among those are socket LGA1150 motherboards for Intel's 4th generation Haswell and 5th generation Broadwell processors, such as the Z97-A/USB 3.1. Asus also has updated BIOSes available for its enthusiast X99 (socket LGA2011) motherboards like the ROG Strix X99 Gaming and ROG Rampage V Edition 10.
The new BIOS releases are all labeled as "beta," which TechPowerup surmises is because they're dated motherboards that, in most cases, are now beyond their warranty coverage.
If you own an Intel 9-series motherboard from Asus, head over to your model's product page and go to Support > Driver & Tools > BIOS & Firmware. To see what BIOS version you're looking for, reference this support document that Asus posted in January—it lists all the motherboards Asus plans to update, broken down by chipset and BIOS version.