With libxcb 1.11 as minimal required version we can:

• enable threaded GL for MESA, see QTBUG-67277
• make xcb-xkb a mandatory dependency avoiding issues described in QTBUG-30911
• avoid performance issues described in QTBUG-46017. Bundled xcb libs don't contain the more modern SHM fd passing APIs, so official binaries that use "-qt-xcb", are shipping with the fix #ifdef-ed out.

CI for Qt 5.13 has:

Ubuntu 18.04 - libxcb 1.13
RHEL 7.4 - libxcb 1.13
openSUSE 15.0 - libxcb 1.13

CI for Qt 5.12 has:

Ubuntu 16.04 - libxcb 1.11
RHEL 7.4 - libxcb 1.13
openSUSE 42.3 - libxcb 1.11
RHEL 6.x - not relevant because it was dropped from supported platforms.

Why 1.11, but not 1.13?

Ubuntu 16.04 LTS will be supported at least until 2020 according to https://www.ubuntu.com/about/release-cycle so there is no real reason to bump minimal required to 1.13 just because we can. But 1.11 is sufficiently easily available even on older distros and having it as minimal required version would fix some known issues. I assume that people still would want to install Qt 5.13 on Ubuntu 16.04 for whatever reasons.