Whilst it seems crazy to me there is no button or even function in the searchable toolbox, creating a script in my file called "Apply Delta to Transform" has worked reasonably for me.
In my case I was happy with the current total transform of the objects, and just wanted to apply the delta transform to the base transform. A script for this is below:
import bpy
for obj in bpy.context.selected_objects:
obj.location += obj.delta_location
obj.delta_location = (0, 0, 0)
obj.rotation_euler.rotate(obj.delta_rotation_euler)
obj.delta_rotation_euler = (0, 0, 0)
obj.scale *= obj.delta_scale
obj.delta_scale = (1, 1, 1)
Open a text editor panel, enter the above script, and click the "Run" button (with the play icon) once you've selected the objects whose transform you want to apply.