- A bull barrel shorter than 20 inches
Depends on what you mean by "bull." A heavy profile 16" barrel is a good compromise between accuracy and portability, IMO.
- A collapsible stock on a rifle with a bull barrel of any length
- A collapsible stock on a rifle with a barrel longer than 18 inches
No AR-15 stock is truly collapsible (buffer tube sets the minimum length at a shootable length of pull). They would be more accurately described as adjustable, and that adjustment can come in handy if your plinking rifle also serves in an HD role. Not everyone can afford one rifle for the bench and another for HD. The 6-position stocks are also handy if people of different stature will be shooting the rifle.
forward assists, deflectors, and dust covers on rifles that are not used in combat, ( may come in handy in some competitions, true)
As mentioned above, a forward assist is nice on a defensive carbine when doing a chamber check and you want to make sure the bolt went back forward all the way. With an AK, you can just push the bolt handle forward, but on a rifle with a non-reciprocating handle, you need a FA for that.
A deflector, I'm not sure what they do. Do they help keep brass out of the face when shooting left handed? If so, that'd be useful.
A dust cover, I'd definitely want. On a windy day with a bit of blowing silt, you'll have a lot of dirt on that oily bolt carrier if you don't keep it covered.
If there was one word I could banish by royal decree from firearms slang, it would be that one. If it has a function, it's not tacticool.
Lights, rails, 1x optics + magnifiers, BUIS, and adjustable stocks aren't tacticool, IMO, because they are perfectly functional.
A 37mm flare launcher, pretty useless, but I can see where a collector trying to construct an authentic-looking M4 might want one.
I'd have to agree with that one as being vestigial and mostly useless. Until someone comes up with something useful to hang on it, anyway. I personally don't like bayonet-lug flashlight mounts because they put the light below the barrel and cast a barrel shadow in the worst possible place.