Hi, i was reading WIKI and it advises that you should remove mods and CC you installed if you gonna add new expansion pack. Does it still apply nowodays, when game isnt being patched anymore? I have base Sims 3 now and i jumped into mods, sliders, sims, clothes, makeup and other CCs which sometimes was tricky to install and i hope i won't have to go through installing all of that again that i have already now when i'll buy some EPs in the future.

If you're patched to the latest version (1.69), or an earlier one but don't plan to update, then no, you don't have to worry about removing mods. CC (hair, objects, patterns etc.) don't need to be removed since they aren't affected by patches -- the only case where this happened was the Pets patch (1.42?) which broke some furniture due to pet routing, but basically all the CC you download now will be made with this in mind (the only exceptions being super old stuff from retired creators).

It depends on your patch level, but I assume you're past 1.42.

Your game will ask to patch itself before installing an EP/SP. If you don't want to forcefully be updated to 1.69, you can get one of these patches that only update you to 1.67. Since 1.69 only adds a compulsory connection to the Internet to play TS3 and doesn't actually fix or add anything, most consider it unnecessary.

Really, the only concern you should have is with mods, specifically tuning ones. The resources affected by those (XML and ITUN) may be updated with patches. That's why it's important to make sure you have the version that matches your patch level -- though, of course, some mods don't need to be updated because the resources they use aren't changed.

Edit: not only tuning mods are affected, core mods should definitely match your patch level. Otherwise things will go badly

For the actual install procedure though, it shouldn't be that complicated. I would pull the entire TS3 game folder out of Documents. After running the game once to test the new pack on the clean folder generated in it's place, then you can put the original game folder back. Overkill perhaps, but still no muss and no fuss over what the pack install and possibly patch checking and install mechanisms themselves might or might not do to our game folder and its contents since they won't be able to find any of it.

I don't recall any reports of Sims 3 having this issue, but Sims 2 occasionally had the bad habit of deleting your CC when you updated or added a new EP. You don't need to delete anything, but moving them away until everything is finished is never a bad idea.

Not go missing exactly, but we've had tons of reports of games refusing to load up, Mods folders not being read, half of content being ignored, etc. after patching (and maybe after pack installs, the procedures seem so similar) to the extent that players have had to rebuild their game folders from scratch. Which it sounds like precisely what the OP is trying to avoid.

I was a rebel back then, never pulled anything out before patching and installing packs and never had any issues on either Mac or Windows. But after helping so many who had, somehow I lost my Daredevil trait around when 1.63 came out and I only had two packs left to add, so started doing it the folder pull way instead. Better late than never.

Honestly, it depends on the mod.
Some are updated to the most current version of the game, others....have fallen off the wagon in maintenance, and might break your game or show up in 10 shades of freaky that you didn't ask for.
If you'd like to make it simpler without having to compromise your game state, I would suggest going through your mods. Things like makeup, hair, and clothes, don't generally explode when you add an EP or higher patch, so they can be kept.
It's the gameplay adjusters that you should really watch for. Things that alter your sims' behavior, their routing, story progression, etc, as those tend to explode often if not updated to the latest version of the game with your newest EP. For those, I would suggest either keeping track of the mod provider, or keeping the files named so that if you do need to look them up and, say, haven't bookmarked their page, it'll be easier for you to get ahold of a newer version of the mod that is compatible with your game.
Let's say you have a gameplay mod whose owner has stopped updating it past the latest version, and you need to obtain a newer version. Sometimes, other people have taken the mod up in their stead, but other times, you won't be so lucky. Absolutely, in those cases, delete the CC that is both incompatible and not able to be updated.
However, if you can obtain a later version of the mod that is compatible with your new game version...just replace the old mod with that newer version.