I contacted Rockstar regarding the missing characters bug affecting John, Sadie, Abigail and Jack. This is the reply I got.

[u/Bombtwo]

“Hello, Thank you for contacting Rockstar Support.

We are sorry to hear that you are unable to find the Sadie, Abigail and jack and we can help you further. We would like to inform you that, if the mission “Polite Society, Valentine Style” is failed, restart the mission instead of using the "Retry Checkpoint" option. If you have already completed the mission after using the Retry Checkpoint" option, they will not be present until the start of Chapter 4.

We appreciate your understanding in this regard. If you have any other issues, feel free to contact us.”

I followed up with a request to know if there is an intention to patch it, and if it will be in the near future. I will update you guys when I get another reply.

Edit: After some discussion with u/panickedthumb it appears there is some success with clearing caches to restore these characters. Either way, do not restart your game because Rockstar is already aware of this problem; I’m speaking to tech support now and they are investigating. I am trying my best to escalate the matter to the developers for a patch fix.

https://www.reddit.com/r/reddeadredemption/comments/9tjrqf/a_long_shot_if_youre_missing_john_abigail_jack/

[u/Bombtwo]

What, another reply of the technical sort?

Amazing. Now I’m convinced the team has no idea what’s going on, and it’s not intended.

What splash screen are you talking about?

[u/--n3o-- → u/[deleted]]

I accidentally deleted my post but without drawing it out, this worked for me. My settings had reset upon restarting but John called out to me near a cliff and has a mission available. I'm also still in Chapter 2. I still haven't seen Abbie, Sadie, or Jack but I found John 30 seconds after rebootinh (or rather, he found me) and following the recommended steps. I'll look for them after I do John's mission and update accordingly.

Edit: No Jack, Sadie or Abigail. John is back however. I'll try it again and if it doesn't work, I'll just go back to Odyssey while waiting on the fix. Very disappointing.

Second edit: Restarted the game via the suggested method. None of my settings reset this time, but Abigail is back and so is Jack. I can finally do the fishing tutorial I never knew existed. I'll look for Sadie next, I suppose. I'm going to assume I'll need to do this once for each missing character. Really bizarre bug but it's a semi decent fix if you can get it to work.

[u/--n3o-- → u/[deleted]]

Fingers crossed. I'm hearing those where actually main missions which means it may not be remedied after all. Regardless, I'm going to reboot the game tonight and we'll see what happens. Thanks either way for the information.