The vast majority of phones out there support GPS and GLONASS (as well as ESA's Galileo), and have for years. That's not new to the iPhone 8 (surely the iPhone has has this before as well, right?)

Using the other constellations helps you see more sats, but it doesn't really help deal with the reflection issue in urban environments. The "big deal" about this chip is that is uses the new L5 signal (and it's equivalent in other constellations). Previously that has only been available in very expensive hardware.

As said, the value here is the dual-frequency support. However, the features listed on Broadcom's site [1] show:

  The BCM47755 can simultaneously receive the following signals:
    GPS L1 C/A
    GLONASS L1
    BeiDou (BDS) B1
    QZSS L1
    Galileo (GAL) E1
    GPS L5
    Galileo E5a
    QZSS L5

[1] https://www.broadcom.com/products/wireless/gnss-gps-socs/bcm...