The short answer to this question is No and I will explain it why. There are actually many people asking this same question on social media,forums and other places so many times that is it illegal to use a dummy ticket as proof for onward travel or visa application. So that's why I want to explain it in detail as I have more than 20 years of experience as a travel agent and also explored half of the world due to my other businesses. So this makes me in a better position to explain you what a dummy ticket is and if its illegal or not.
what is a dummy ticket ? then will explain its legality.
A dummy ticket is actually a reservation of a seat for a specific time period after which your reservation gets cancelled or you will have to pay full amount before that time period to confirm your ticket. Mostly you will get a dummy ticket for 2 days period and within this time you will need to pay full amount of the ticket otherwise your reservation will automatically be cancelled. you also will be charge from $10 to $20 for this dummy ticket. so it didn't came for free.
Can I verify my pnr online for my dummy ticket ?
Yes, off-course you can verify your reservation or pnr number online within these 2 days or any specific time period depends on dummy ticket provider. After this time period you will not be able to verify it as it will be cancelled automatically if you didn't paid full amount of it.
How we say dummy ticket is not illegal?
Dummy ticket is not illegal and its because there are many countries which asks for reservation only as a proof of onward travel or visa application. you will need to look what that country is asking for. If you country is asking for a reservation only in their requirements then its this dummy ticket that you will need to provide to them. One other thing which you need to care about is that, if you don't want to pay full amount of your dummy ticket then make sure to generate this ticket in good time so that you can show it to the officer before it gets expired.
will it be considered as illegal if my reservation got cancelled?
No, it usually happens and majority of the countries don't have much strict rules to check every ticket online so if you are within this range then you might be able to succeed with your expired dummy ticket as well but if in case an officer check it online and there were no record due to expiration of your dummy ticket then the officer will not consider it as illegal because he knows that reservations may get cancelled. So in this case you will need to book a reservation again with a new dummy ticket and then you will be able to fly to your destination.
if a country required list says booking only and not mentioned reservation:
If reservation is not mentioned and only booking is mentioned then it clearly says that a reservation or a dummy ticket will not work for you. you will need to buy a full confirm ticket which will costs you allot but in this scenario dummy ticket will not be helpful. Its still not illegal but they will ask you book and confirm it which is their requirements that you have to fulfill.
I think this explanation will hopefully help you understand that what a dummy ticket actually is, where you can use it and where you cannot.