The table of Qt versions shows which versions have been used in Wireshark installers for Windows and macOS. Determining the versions available on specific Linux distributions is a bit more of an artform but can be gleaned from the tables further down.
Ah, I see, yes I misread the table and interpreted the bold versions as minimum requirements. I just took a look and updated the RHEL/CentOS and SLES information. Looking at the rest of the tables, Debian Jesse just went end of support, so Ubuntu xenial (16.04LTS, end of support April 2021) being at 5.5.x is I think the oldest QT in a still officially supported distribution on that page. After that there's several at 5.6.x.
I reckon that QT 5.9 is fine for every distro's most recent LTS release, and 5.6 is fine for every distro's N-1 LTS release. Personally I'd be fine with moving master to 5.9 (which also settles requiring C+11), but I could understand 5.6.