Not that it matters (it doesn't), but I don't have an exact number. I know of a few off the top of my head. Here's a quick and dirty little list I threw together of some things I can name off the top of my head.
- Rollermine Spawner - Uses Rollermine model, rollermine sounds
- Roll The Dice - Looking at the code, I see some HL2 button sounds are used
- Dr. Hax - Old plugin, uses computer monitor model from HL2, voice clip of guy saying "HACKS!". Looks unmaintained, but I've got a working new syntax version I just haven't released yet.
- Various "pet" plugins for TF2 - I've got several different copies of source code for several versions of a "pet" plugin including Golden Machinegun's Pet plugin, a newer pet plugin from AlliedMods, and one I've made. They all use HL2 models for the pets, and HL2 sounds for the respective sounds for certain pet "actions" like jumping/taunting.
- MasterOfTheXP's TF2 Sandbox plugin, and my version of a TF2 sandbox (aka prop building) plugin. These both use every HL2 model.
- SourceMod itself - Menu sounds are HL2 sounds, though they can be changed in the configs.
- "gibshooter" entity - Appears in all Source engine games. Shoots out HL2 gibs like a skull. This would probably be permanently broken in TF2 if HL2 content was removed. This does work in TF2 (I tested it fairly recently).
Again, it doesn't matter how many plugins use the files. The point is the files are used. Valve isn't going to remove them because of this Git issue which is why I deleted my prior comments over there. It's pointless even discussing this as I've now realized because it's been what, 10 years or more that TF2 has been around now? This is Valve we're talking about, they won't be touching anything to do with the shared directory. That's why it's been dubbed "shared", because it's shared by HL2 and TF2. Some stock TF2 maps even use HL2 assets so this silly suggestion of removing the files would break those too.
The key word is "ideally". You aren't the first to suggest this, and if the files remain in, you probably won't be the last. TF2 is like, 10 years old. It's too late at this point.