A new Crash Team Rumble post confirms Mar. 4 will be the last content update for the online only experience.
According to the notification, on Mar. 4 all players will gain access to a free 500-tier battle pass. This includes 104 new items, plus everything from the previous three seasons, and other event rewards.
Unfortunately, the notification does not mention if Crash Team Rumble will be delisted, but it does confirm Crash Coins can no longer be purchased. There is also no explanation of how this battle pass will work, though hopefully previously unlocked rewards can be skipped.
Finally, for anyone worried about the trophy/achievements can take solace in knowing population is not an issue. Spots not filled by human players will have AI bots instead. These enemies are extremely dumb, which will make certain tasks easier, with other tasks being harder.