There is still a timer on them. At least, there has been for me. Maybe at a certain threat level that changes, but I don't have that issue yet.
The timers have been removed. What you're talking about is the countdown to when other sentinels arrive. I'm talking about a timer that used to countdown to when the sentinels would actually 'call' for back up. It was a much better system and varied slightly from planet to planet. Graphically, it showed as pulsing rings emanating from the sentinels. This gave you a window of opportunity, so rather than just fighting and knowing you are just making matters worse, you could kill the two sentinels (at the time) as quickly as possible BEFORE the timer had completed and more were called.
What they should have done is what I suggested in a previous post because right now it's not worth the hassle. There is absolutely no reason to kill sentinels unless you just like to shoot stuff and want to show off your latest gun.
2: Reinstate the timers on the sentinels. I can only think of one reason they've done this, and that's the please the 'pew-pew' crowd. All they had to do is tweak the old system a little and they could have pleased both the explorers (which there are many and 'were' once the mainstay of the NMS community) and the battle orientated. It was ALWAYS the case that the timers varied and you'd sometimes have less or more time to deal with the sentinels before they called for back up. Now you don't really have an option unless you fancy killing every sentinel that comes your way. All you can do is run, which I see nearly everyone do on Youtube. Before this change, even explorers would take on a sentinel or two.
Adjust the sentinels and give them personalities, depending on the planet. Some could have 1 - 3 second timers, giving us the same result we're seeing now. Others could have 5 - 10 second timers, making it an urgent matter to kill them as soon as possible, while others could just be as they've always been. The change has removed that moment of panic you had when you wasn't sure you could deal with them fast enough. There's no point in killing them now.
Some examples of possible sentinel personalities. These (or whatever they decided to call them) could be information added to your scanners, giving us a very good reason to actually scan the sentinels:
Fickle: These sentinels vary in the time you get. It could be anywhere within a range of 1 - 20 seconds, depending on how they felt that day. That would add a moment of regret or relief once you see how fast the timer is moving.
Bully: These sentinels only ever give you 1 - 3 seconds before they call for back up. They're to be avoided at all cost unless you decide it's the day the worm turns.
Balanced: These sentinels give you between 10 - 20 seconds before calling calling for back up and are the pride of the Atlas.
Aggressive: These already exist, and I hate the little blighters.