With removal of support for Qt4 not far away, opening this for any thoughts on setting a minimum required qt5 version. It could be quite possible for us to support Qt 5.0+.

Qt4 support was removed in #13458.
The current minimum required Qt is 5.2. Set in #14725.

Qt Versions:

5.12 (LTS) Supported for 3 years post release.
5.11 Supported until May 2019
5.10 Supported until Dec 2018 (Archived).
5.9 (LTS) Supported until May 31, 2020.
5.6 (LTS) Supported until Mar. 16, 2019 (will end around the same time as the v0.18.0 release).

Any other releases older than 5.10 are no longer supported (by Qt).

Qt releases seem to be getting more frequent, and in some cases more aggressive about dropping support for OS versions. i.e macOS >10.12 is required for Qt 5.12.

Qt Feature Usage:

There are the following usages of QT_VERSION (excluding bitcoin_qt.m4) in the code:

• QT_VERSION >= 0x050600 - 5.6+ Enables high-DPI scaling on supported platforms.

• QT_VERSION >= 0x050500 - 5.5+ Disable SSL and use TLS 1.0+.

• QT_VERSION < 0x050400 // Import AccessibleFactory plugin.

• QT_VERSION >= 0x050300 - 5.3+ tests using QVERIFY_EXCEPTION_THROWN.

• QT_VERSION >= 0x050200, QT_VERSION >= 0x050200 - 5.2+ Returns the most appropriate system font.

• QT_VERSION >= 0x050200 - Set this menu to be the dock menu available by option-clicking on the application dock icon. macOS only.

• QT_VERSION > 0x050100 - 5.1+ Support for high-dpi pixmaps.

• QT_VERSION > 0x050100, QT_VERSION > 0x050100 - Sets the device pixel ratio for the splash-screen.

• QT_VERSION >= 0x050000 - Determine if a certificate is blacklisted. Removed in gui: Drop qt4 support #13458.