If Jacob Trouba does not report to camp with the rest of his Winnipeg Jets teammates because he does not have a contract, do not call it a holdout. It is not a holdout. A holdout occurs when a player has a contract, but does not want to honour the terms of that contract or wants those terms to be changed. An example of this is Alexei Yashin with the Ottawa Senators. Jacob Trouba does not have a contract to holdout with. Trouba is simply a player without a contract (and could remain that way up to and including training camp and the regular season).
Trouba has absolutely no contractual obligation to show up at camp. In fact he has no contract and could sign anywhere else right now. He is a restricted free-agent, which means he is a free-agent but him signing with another team would require the team that signs him to compensate the Jets as prescribed at the beginning of the off-season. This does not mean that Trouba is not allowed to sign with a different league outside of the NHL. In fact, that could work to his advantage as he would be able to stay in playing shape, but he would also have to clear waivers if he is signed by a team other than the Jets (call this the Ryan O’Reilly Rule). This means that if he chooses to go over to Europe, Trouba would be limiting his options to the Jets, even if any team can technically sign him.
There are other young players who have done the same thing and been without a contract at the start of camp. Drew Doughty and P.K. Subban are both defencemen who have gone through this recently and both turned out fine. Johnny Gaudreau is also without a contract and it does not sound like him and the Calgary Flames are any closer to a contract than they were at the start of summer.
In short: Jacob Trouba cannot holdout if he does not have a contract. He can sign and play in Europe, but that kind of limits who will want to sign him in North America because he would have to go through waivers for any team that is not the Jets. He is not the only young player who is going through this. He will probably sign with the Jets at some point in the future. Until then he is simply a restricted free agent without a contract.