I think the idea here is good, but comparing the string values is an unreliable way to implement it. For example, the check would fail for a hypothetical Qt 6.10 because a string starting with "6.1" sorts before one starting with "6.6".

I would instead use a tuple in appinfo.py:

required_qt_version = (6, 6)

And write the check using QVersionInfo like so:

from PyQt6.QtCore import QLibraryInfo, QVersionNumber
v = QLibraryInfo.version()
r = QVersionNumber(*appinfo.required_qt_version)
if v < r:
    error("Qt version is too old.",
        f"Frescobaldi is started with Qt {v.toString()} \
but requires at least version {r.toString()}.")

This ensures the version is processed numerically as intended.

Looks good to me.