The fate of Trevor Philips after GTA Online's latest update indicates that he's still alive and well. However, players seeking to find out everything that happened should know that it's a little more complicated. Note that everything discussed here was relevant by the San Andreas Mercenaries update. If this character returns in a future update, it won't be referenced here.
Nonetheless, there are still several DLCs that hint at what Trevor Philips has been up to in the past few years. Remember, Grand Theft Auto 5 took place in 2013. All recent GTA Online updates arrived in the current years, meaning it is possible to determine this character's current status within the series.
Trevor Philips became a lifestyle coach sometime in GTA Online
The Diamond Casino & Resort DLC had a mission known as Casino - Strong Arm Tactics, referencing Tao Cheng's survival in The Third Way mission. That meant Trevor Philips was canonly alive after Grand Theft Auto 5 ended.
Smuggler's Run, an earlier update, had Ron Jakowski state the following in a cutscene after the player purchased a Hangar:
"Tre... uh, my best friend... he's supposed to protect me forever... then he goes all Vinewood on me... he's too important for Ron now... he's a guru... he's a lifestyle coach... I don't know what he is..."
That means four years after the events of Grand Theft Auto 5, Trevor Philips decided to become a guru of some kind. It's a bit vague, but that's everything known about the former protagonist's new career.
It wouldn't be until 2021 that GTA Online players would hear about this character again. The Contract DLC has a security guard that can make a random comment about a number of characters. One thing he can say is this:
"Hey, this crazy dude swung by earlier, dressed like a hobo. Like really on edge, he swore him and Mr. Clinton used to work together. Got really angry when I said he wasn't available. I had to stop him from sh*tting on the desk. Seriously. If you get a minute with Mr. Clinton, would you let him know?"
The only character this description would fit would be Trevor Philips. That meant he was still alive by 2021.
Los Santos Drug Wars was the last major GTA Online update to reference Trevor Philips. That DLC basically showed how all of his old businesses are no longer functional, which is why Dax and his friends took over. That was the last time he was referenced.
The GTA San Andreas Mercenaries update did not hold any further details of what this character could be up to in 2023 onward.
Other notes about Trevor Philips in GTA Online
It is worth mentioning that this character also gave GTA Online protagonists the Series A Funding heist and some minor Contact Missions. However, all those actions canonly took place before Grand Theft Auto 5, making them less relevant for determining his fate in the modern era.
He could always appear in a future update, which could introduce new content, patch bugs like GTA Online money glitches, and more.