GTA Online has had its Survival mode for quite a while now and it appears that Rockstar Games are looking to add it to Red Dead Online as well.
Survival mode in GTAO might be somewhat confusing due to the naming of various modes in various games. Usually, a survival mode means scraping materials and ingredients together to build a shack that will get raided by a group eventually but in the Rockstar titles, it refers to what we know as Horde mode.
Several waves of enemies will run at the players, with each wave being more difficult than the previous one. Should the team survive, they will complete the activity and reap better rewards.
Survival has been around in GTAO for a while but RDO still doesn't have it. That stands to change with the Blood Money era apparently since various data mining findings point to it being added as drip-fed content . This turned out to be correct with the homestead robberies previously, which now became part of the Blood Money crimes that can award Capitale.
Another dig in the available data revealed that the mode will be named Call to Arms but it's currently unclear if this is the final decision or a placeholder title. Considering that it will actually be something that RDO didn't already have in a different form, it may be safe to assume Call to Arms will be well-received by the players.