Can salted butter be used to make lemon curd?
Often, at least in the US, recipes will specifically call for unsalted butter, then call for salt to be added to the recipe — which causes many to scratch their heads.

There are three main uses that come to my mind for salt. One is to impart a salty taste.  The usage in butter is primarily a secondary one, and that is as a preservative.  The third is as a flavor enhancer (which can also apply to butter, but to my knowledge is not the primary reason there).  Many recipes that call for a small amount of salt are using it strictly as a flavor enhancer.  The intent is to highlight other flavors without actually tasting the salt.

So why call out unsalted, then add salt?  Two reasons that I know of.  The amount of salt in salted butter varies.  If you read the label carefully and do the math, you might get a rough idea, but in general by just picking up the butter you do not know how much salt is in there.  Some may have enough to taste, others you may not notice.  By using unsalted, you are in control of how much salt is added and can even adjust to taste as individuals who practice a lower-salt diet tend to taste it at lower amounts.  Higher-salt diets tend to become numbed to the taste so do not notice it at lower amounts.  This contributed to the older tendency to call for a "pinch" and a pinch varied from person to person according to their own sense of taste.

The second reason is more subtle.  Salt in butter is a preservative.  Since it has salt, it tends to hold up longer in storage. Since it holds up longer, there is at least a perception that salted butter may be older.  So calling for unsalted by some cooks and authors can be like saying "use fresh butter, not old."  It is an old tendency, and many may not even realize that is an origin, even if subtly implied.

Now, not knowing your curd recipe, it may not call for added salt.  I come from a high-salt background, so it is unlikely I would taste the difference, while others might.  In the case of your particular recipe, there may be a tendency for at least some people to actually taste the salt, especially given that the amount of salt in the butter may vary by maker.