Invisible only doesn;t show the notification, but play time is still counted on your profile. If you check a profile and you see the latest game's play time increasing over time but they offline, it means they invisible.
Right click a game (or press Start in BigPicture/GamingMode), select Properties. Go to the Privacy tab, and there you can hide the game from everywhere but yourself. The game will only be visible in your own library when logged in, and people won't see you playing that game when you run it (you appear Online but not playing anything, your nickname in friends list remains blue). You also make the game invisible in your profile, both on the main page with the play time and on the full games list.
You can also do it before purchase, now every time you put a game in the cart you are asked if you wanna make it remain public or hide it.
If we talking about modern Android, good to know. I used to run a launcher on a modded Samsung Galaxy Young (CyanogenMod11) that had a fixed notification option to prevent Android from killing it (the device had 512MB of RAM).
Clash of Clans is kinda safe in this, nobody cares anymore if they get attacked as there are now so many ways to farm resources that got added after the game started dwelling in the player count.
SSUPII @ SSUPII @sopuli.xyz Posts 118Comments 744Joined 2 yr. ago 
Why bother with a grill or oven? Just stick some sparklers in your fish and light them up—dinner and fireworks all in one! Comes out perfectly cooked.