This change is undocumented in the snapshot changelog, so I have to imagine this was an accidental breakage when updating the UI that was never caught due to nobody on the team using a resource pack with an overlay. Attached is a comparison image between the two snapshots, As you can see, 24w09a no longer uses the gradient defined in the texture. Also attached is a pack which should turn the main menu into a static image of rainbow-colored wool, for easy testing. The breaking of panorama_overlay.png removes the opportunity for static backgrounds to be used on the title screen, which is quite unfortunate if it sticks.
Linked issues
relates to 18
MC-278413
Chat background cannot be retextured
Open
MC-264571
Both standing and wall sign GUI backgrounds cannot be retextured independantly from the block
Resolved
MC-270255
Item slot disabled effect cannot be retextured
Open
MC-270258
Durability bar cannot be retextured
Open
MC-270260
Bundle fullness bar cannot be retextured
Open
13 more links
Can confirm.