TL;DR: Backup 1 file: GTA5.exe, (GTAVLauncher.exe, PlayGTAV.exe optionally) from the game/installation folder, and definitely keep a backup of Update.rpf (from Mods folder, not game). The first 3 files listed will get overwritten by each update.
Optional, but highly recommended: backup ScriptHookV.dll. This is the file you will be using as you wait, so it's not necessary for now. However after SH is updated, when you have updated to the new DLC, it will allow you to revert back - if you don't like the update or it causes problems for you. That's it, that's all you really need to do.
See also: UPDATE TO NEW GTA5 WITH NEW SH5 RELEASE
The Long and Short
The rest of this post is for those who are curious or to fill in some information gaps. For example, at the recent tuner update it seems only GTA5.exe was required from the .exe files, but who knows how the next update will behave - better play safe and back them all up. Note that the update will not overwrite any of your own files such as ScriptHookV.dll. In fact, it won't overwrite any modded files unless you modded game files directly - but it's good practice to backup your mod files as well, particularly Update.rpf from Mods folder.
R* doesn't give a damn about SP but they still provide DLC for GTAO. Every time they do, it potentially breaks modded games. Generally the fix is just a Scripthook update, but lately it has also caused other issues leading to crashes and requiring changes to gameconfig.xml files.
@WGotch07 has given us a warning that another update is coming. This is what I suggest you do to prepare for the next update. This process is just to allow you to continue playing in SP (Sandbox, free roam, etc) while waiting for AB (Alexander Blade) to update SH. Remember to update SH from official site only.
The good news: An update will not make any changes to any mod files anywhere - as long as you used a mods folder. None of the mod files (scripts, asi, dll, etc) in other folders will be impacted. So nothing lost unless you screw up, for example if you installed in game folders.
Make backup copies of GTA5.exe, GTAVLauncher.exe, and PlayGTAV.exe. These files are in the root folder of your game. From the last update, I found only GTA5.exe was needed, but better to err on the safe side and backup more than less - these files are small and easy to backup.
Also backup, from your MODS folder, the file called update.rpf. I'm assuming here you were wise enough NOT to make changes directly to your games folder. You can also backup the update.rpf from your game folder but that's optional and not necessary.
That's it, that's all you need to backup. Now what? This is the Steam process, it should be similar for other versions except for some launchers continually trying to update.
2. REVERT TO PREVIOUS GTA5
After the game updates, you need to copy the 3 .exe files back to your GTA5 root folder to overwrite the new ones. That's all you need to do.
Now here's the fun part, if you go in OpenIV you will get this alert box saying your update.rpf is outdated. This is why we backed up update.rpf because it contains your dlclist.xml and other user customized files. You don't need to do what OpenIV says. In fact you don't want to, not until the new SHV is released, and remember that you need to keep your old update.rpf because it contains files you will need later. If you let OpenIV replace update.rpf you lose your dlclist.xml and gameconfig.xml, etc.
Each update is different so WATCH THIS THREAD FOR ADVICE AFTER THE UPDATE. This will include updating the .exe files and adding the new SH etc.
3. UPDATE TO NEW GTA5 WITH NEW SH5
Once the new version of ScriptHook is available here is what you might want to do.