Why "Export as Python script" button is unaccessible in QGIS 3.0/3.1 Modeler by default?
This was intentionally disabled by commit 4f2a464
[processing] remove obsolete code and disable exporting models as script until we update it so support new scripts format
@alexbruy alexbruy committed on 2 Feb
So re-enabling it yourself won't be of any use as the output code won't be usable in QGIS 3x.
Check https://gis.stackexchange.com/a/313547/187 for updates post 3.1!
Spoiler: it's back!