Is this an Appeal to Consequences fallacy?

Appeal to consequences applies when you argue that the truth/falsity of a statement is based on the consequences it would have.

E.g.

"My grandmother cannot be dying from cancer, because it would break up the family."

"I'm certain to win the lottery, because it would help my wife and I pay off the mortgage on our house."

It's basically a non sequitur in the sense that a statement's truth value is determined based on whether it has positive or negative consequences, when that is actually irrelevant to whether it is true or not.

It doesn't apply to moral judgements (e.g. "we should restrict free speech because if we do not, we risk a far-right resurgence") because we are not affirming/denying truth (descriptive), we are suggesting behaviour (prescriptive).

This is a mere unsupported claim, possibly an opinion.

This is more of claim than an argument. As a claim, there is no fallacy because we don't have access to the reasoning. It sounds like hyperbole , as well as a claim that would either be factually correct or incorrect.

I'm not seeing any fallacy here. IMO to discuss the potential consequences of putting a particular law into place is reasonable. Laws are generally put in place with the aim of bringing about more good than bad (in a legal sense)  so in your example consideration of the consequences is relevant. 

If the arguer could demonstrate that untaxed sex work income could lead to people exploiting the law to launder illegitimate earnings, that would be a reasonable concern to mention.

I believe it is an example of appeal to consequences, at least tacitly. It is implied that not taxing income from sex work is bad because money laundering (the consequence) is bad.