With what backend? The rubber band should be alternating black and white in most (but not all in 3.5.3, IIRC.)

IIRC it's wx which used that colored rectangle previously. Currently it doesn't draw the white dashes; seems straightforward enough to add that. Looking at other backends, I believe tk and macos also need the improvement.
Marking as good first issue (but medium as you need to fiddle with GUI toolkits (and ObjC for the macos backend) -- it's not necessary for a PR to handle all backends at once).

The MacOS-backend has a separate issue with a bit of possibly useful details: #23777

For wx one can add the following line

            drawDC.DrawLineList(
                [(x0, y0, x1, y0), (x1, y0, x1, y1),
                 (x0, y0, x0, y1), (x0, y1, x1, y1)],
                wx.Pen('WHITE', 1, wx.PENSTYLE_SOLID))

before

wx does not support 0-length dashes, so one cannot start with a gap (as far as I can tell).

Bonuses:

• Only create the point-list once per call
• Only create the pens once per CanvasBase instance.

Thank you for your consideration of this issue. I tested on Windows 10 with matplotlib 3.5.3/WxAgg and TkAgg. The two seem slightly different:
WxAgg => black dashed line
TkAgg => black solid line
The black and white line is also nice.