There was a reddit post a week or two ago that was having trouble keeping their Link on while watching youtube. I posted a fix in there but forgot to make it more public for others with the same issue to find. So if your Steam Link is automatically turning off / hibernating / standby / shutdown / suspending whatever you want to call it there is a temporary fix until Valve adds a setting for it.
The steam link is set to enter a suspended state after 15 minutes of inactivity by default, the defaults can be changed with two files though. Create two files, name one `suspend_timeout_idle.txt` and the other `suspend_timeout_interactive.txt`. These two files will contain the number of seconds before the Link will ask if you're still using the device, and the other file is the time before the Link will auto-standby without asking. I still haven't entirely figured out why there's two files yet but by default both of them are set to 900 seconds so I just keep them the same value with no issues. It seems to use the interactive one first to ask the user and then the idle one has a clamped minimum of ~60 seconds before it will auto-standby; after asking the user if they're still there.
Then in those two files (suspend_timeout_idle.txt and suspend_timeout_interactive.txt) put the number of seconds you want your Link to be on before it goes into standby. I use 7200 for 2 hours but you can use any integer value, if you want you can even use 43200 for 12 hours; doesn't matter.
Then put those two files onto a pendrive in these folders /steamlink/config/system/ So you'll have three folders, the two txt files need to go into the system folder, which is in the config folder, which is in the steamlink folder.
From there just un-plug your Link and insert the pendrive. Then re-plug in your Link and after it boots it should read the two files and stop standing by after 15 minutes. This should persist permanently unless you do a factory reset or a future update removes the support for it or the files themselves.