The explanation is that you have the subjunctive followed by the conditional. Vouloir que does trigger the subjunctive, which is here "aiment" and you missed it because in " aimer " the subj. is the same as the present ind. The bit after the comma is a conditional statement, you are saying that IF you were to do as the French don't like THEN you WOULD be breaking a taboo. Conditional statement.I realise this is a big ask, but it’s going to bug me a lot if I can’t find the answer, so I wonder if any French speakers feel up to answering this question. I had a conversation about the issue once here with @Steve 57 and @PsB
On the radio this morning - France Culture, so I guess it’s good French.
Le cliché veut que les Français n'aiment pas parler d'argent, que ce serait tabou.
The question, if you haven’t already guessed, is why serait and not soit?
It's actually the same in English.
I wish that I were (subjunctive) taller, if I were (conditional, here it comes) then I WOULD BE (conditional, ie if X applied them Y would follow) more attractive.