This script can't yet handle survivors that spawn in the middle of the round, so during a chapter, not recommended to add/remove survivors or change the survivor count. CST only auto records survivors to a list at the start of the map.
(unless you want to disable auto takeover and do it manually via !takeover chat command)

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In singleplayer:
VScript Survivor Manager (VSSM) by Shadowysn is recommended.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3006216183
It also fixes many bugs (for example: fixUpgradePacks, restoreExtraSurvsItemsOnTransition, fixDefibrillator, etc...)
I found that VSSM Defib fix sometimes work, sometimes doesn't. I think it starts to fail when progressing more into the chapters of a campaign.

Again, only change survivor count using the settings file and not the chat commands.

VSSM settings:
https://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/3006216183/7222135514705252606/

Not recommended for multiplayer yet, because of a bug.

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For multiplayer:

You can use sourcemod plugins like ABM or Superversus.
https://forums.alliedmods.net/showthread.php?t=291562
https://gitlab.com/vbgunz/ABM

Maybe need to disable identity fix? Idk needs more testing.
Also recommend defib fix plugin
https://forums.alliedmods.net/showthread.php?p=2647018

Just keep in mind that other players without the custom assets will just see ERROR.mdl.

Going idle may move players into spectator team, and need to do "jointeam 2" in console to join back in. The ABM plugin helps to prevent this, I think

BigFellaBubsy found that the sourcemod "[L4D2] Vocalize Based Model.smx" plugin makes some custom survivors use wrong voice.