No, this is not slang. Slang is non-standard words or phrases which we deliberately choose to use in certain situations or with certain groups of people. Saying "walkin' " instead of "walking" is what naturally happens when some people are speaking in a fast, relaxed way. Note that the apostrophe at the end of the word indicates that there is a letter missing. This way of speaking is a common feature of many variants of English, particularly that of people in the Southern US and also some African-American communities. It is not normal to write these words in this way unless the writing is imitating speech, such as in song lyrics and slogans. You should never use them in ordinary writing, however informal. Examples of advertising slogans imitating this style of speech are 'Finger lickin' good' from KFC and 'I'm lovin' it' from McDonald's. This form is deliberately chosen to appear relaxed, informal and unpretentious, and to appeal to certain socioeconomic groups.

Just in case it wasn't clear enough from other answers: This is grammatically incorrect, but it is how many natives speak. It shouldn't be written this way. I haven't met anyone who write in this way (with no irony)

Not slang. Just a casual, natural, common way of speaking. Wanna, gonna, etc...

Too lazy to add the 'g' lol

Yes also just lazy vernacular