CSS extensions. When to use, how to figure out which to use for...etc.

I am struggling a bit with trying to figure out how to know what CSS properties need extensions.

I found a handy site called autoprefixer.

https://autoprefixer.github.io

that you paste your code in, it finds any problems, which for me caught a single missed semi-colon.

Then, when your css is corrected it gives you back your code with the extensions added in.

However, as I am checking it out line by line to see what was done, I found a section that is below. In that section it uses terms I wasn't familiar with so I looked them up and, got the warning that will follow below.

.flipOverlay, .slideOverlay, .adOverlay {
    color: #fff;
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0; 
    left: 0;
    padding-left: 20px;
    padding-right: 20px;
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;  /*necessary to center text within overlay*/
    -webkit-box-orient: vertical;
    -webkit-box-direction: normal;
        -ms-flex-direction: column;
            flex-direction: column;
    -webkit-box-pack: center;
        -ms-flex-pack: center;
            justify-content: center;
    -webkit-box-align: center;
        -ms-flex-align: center;
            align-items: center;
    background: rgba(0,0,0, .5);
}

Here are a couple of the warnings as found on MDN

for box-orient: Warning: This is a property of the original CSS Flexible Box Layout Module draft, and has been replaced in newer drafts.

for box-direction: Warning: This property reflects an old version of the CSS Flexible Box Layout Module standard. The -moz-box-direction will only be used for XUL while the previous standard box-direction has been replaced by flex-direction.

My question is how do I know when to leave things in and when they are no longer necessary. The autoprefixer from what I have been able to find online states that it is updated to reflect current information so when a prefix is no longer needed, it will automatically take it out of your code and fix it properly. I found some charts that list things that need prefixes, I am just unsure what is actually current and how to find that information out.

Thanks!