Ignoring folder pattern in deja-dup
Solution 1:
Unfortunately, Déjà Dup doesn't support that. The problem has been discussed for more than a decade ¹ ², yet it is stuck at zero progress because devs seem to expect a perfect UI design proposed to solve all use cases—even for beginner users which probably don't have this problem—from the get go, instead of just providing a simple glob pattern input method which will solve it for most users.
The best workaround in my opinion, although admittedly hacky, is a solution by Ian Puleston delegating to a shell script the expansion of glob patterns at backup run time.