Not having a steady source of income shouldn't be a hindrance for investing in bitcoin so from you are saying, even if one has a discretionary income so long as they don't have steady source of income they can't invest in bitcoin. This is wrong, because one can be investing in bitcoin using discretionary income while working to stabilise there income. So to me the most important thing is if the person has a discretionary income to invest with and as time goes by , stabilise his income
Discretionary income and basic knowledge are important for investment. If you have it, then you can definitely enter into investment. In this case, the source can be somewhat important. If someone means a stable source of income, from which you are guaranteed to earn for a long time. Then I can say that stable income can be important for investment (for some people). If you have a source of income that can stop at any time and there is uncertainty about getting the next source of income, then I cannot advise such a person to invest in such an unstable source of income.
The amount of income from such an income source may be high, when you start earning, discretionary money is not difficult for you. But what do you do when you do not have a source of income? You may talk about important funds, but I think investing with such an unstable source of income is risky. Because it is difficult to manage life with such an uncertain source of income. Even if you have a lot of money today, you may be a pauper tomorrow.
You should be sure about the longevity of your income source so that it is easy for you to take the necessary steps. If you have an idea about how long your income source will last and if you can cope with it, then I can suggest starting investing even with such an unstable income source. In this case, the income source needs to be discretionary.