Most likely, before support ends, IO is going to provide an option for anything that is still available in the game (suits, weapons, etc.) to become available for anyone who decides to pick up the game for the first time after support has ended.
Whether this means ETs will have a permanent presence through Arcade mode down the road, or simply unlocking them through the completion of certain escalations or Featured Contracts is unknown.
Most likely it will be through Arcade mode, as ETs already presented in there are likely to get at least a second release with different complications so that players can choose which version they want, and unlock things that way.
Or IO may just make the remaining items unlockable through completing certain challenges already presented in the base game through an update, and those who have already beaten those challenges (i.e. reaching level 20 in a particular map) will immediately have those items after that update.
Hell, they may even give out one last DLC package of unlockable items all in one go for a purchasable fee. People’d be pissed, but it would be the final release for the game before they stop working on it, why would they care how mad anyone is at that point; take it or go without your precious suits forever, we don’t care anymore, we’re about to start making the next game. I doubt they’d do that, but the option is there.
Either way, eventually, I’m sure everyone will have access to everything that is still available in the game, or a chance to get access anyway. At this point, it’s just a matter of waiting.
The real question is: will IO make things unlockable in offline mode? If so, all of the above is possible. If not, that’s when it becomes iffy.