You can say stuff like:
"Not enough! Haha."
"I'm currently looking for a job in 'this' salary range."
"I don't think my current salary is indicative of my market value, hence why I am looking for other opportunities."
Just keep deflecting, keeping it light-hearted if possible. I've had some tenuous conversations with recruiters as they tried to get my salary out of me, but how is that productive for them? All they did was run off a potential recruit and shrink their candidate pool. They'll say things like "well we don't want to be wasting a companies time if their offer isn't going to be within your range". And you just reply with "Well, I'm looking for positions in this salary range, so assuming everything else is positive with the opportunity, I'll be excited for a salary close to that".
Again, a recruiter gets paid by the employer. All they care about is getting a warm body in that seat. They will say "The more you get paid, the more I get paid!" but that's just mental manipulation. If they hire you at $100k, they probably get a $100k fee from the hiring company. That is then split, I don't know, 50/50 with their firm. So they walk away with $50k. Who knows, I'm making this up. But say they do. Well if they negotiate an extra $10k for you (so now $110k) they are taking home $55k instead of $50k. So, correct, the more you make, the more they make. But there's no way in hell they'll risk LOSING that $50k fee by stalling and trying to run up the price. Their incentive is to close the deal as quickly as they can. The longer it takes, the more likely one side or the other gets cold feet and they end up with nothing.
That being said, remember, it's all a negotiating tactic and may not be worth it if you really don't have a lot of room to negotiate. If you REALLY want that job, then so what if you sacrifice a chance at an extra $5k? Go ahead and tell them. I wouldn't suggest lying, but if you say "I'm currently in the range of XX and XX" then you leave yourself plenty of flexibility while giving them enough of an answer to check their box (again, this is if you DO give them your salary).
Someone above mentioned a sales position requiring a pay stub. I could maybe see this as validation of what you put on your resume from a commission standpoint? But I couldn't fathom this method flies for salaried positions.
Edit: In this post, I'm referring to third party recruiting firms when I say a "recruiter".