For example, 「好きですよ」would turn into「好きっすよ」and, 「電話ですよ」would be「電話っすよ」. It has happened for easier and slacker pronunciation. 「マジっすよ」means sort of ‘(I’m/Something is) positive/ no kidding’. This 「っすよ」is restricted in a dual way. 1. too formal when speaking to your peer or someone lower. because it has 「です」. 2. too casual when speaking to somebody superior, because it is sloppier pronunciation. This leads to you use this only to someone older or upper, but actually very close at heart. You might want to save it until you get pretty fluent. ^^

Yes. Means "is", it is slang. Young people will use. マジ＝Seriously,not a joke. When someone you've been asked "it is true?" or “Is it a joke?” And you used if you answer "It is true" “Seriously” in an informal feel.

It looks like a slang. But we usually use it for elder person.