The people who autox still rarely come here, and those of us who do tend to answer more focused questions. Feel free to update things though and I bet Bill will edit the main posts you point out.

Is there a site/forum where WRX/STI owners tend to go for autox advice? I may post something up just so there a little bit more relevant information. Some of those classes don't even exist anymore haha.

Classes

Which class you are running in is decided by the car model you drive, plus a category depending on the modifications you make to your car. With increasing levels of modifications, a logical progression is from Street category (cars with no or minimal modifications) over Street Touring, Street Prepared to Street Modified. There are categories beyond Street Modified (Prepared, Modified) that are not within the scope of this document.

The possible classes for a 2.5 RS are:
Street: GS
Street Touring: STS
Street Prepared: DSP
Street Modified: SM

The possible classes for a WRX are:
Street: 2009-21 - DS, 2002-2008 - GSStreet Touring: STH
Street Prepared: ASP
Street Modified: SM

The possible classes for an STi are:
Street: Type RA 2018 - BS, 2004-2021 - DS
Street Touring: STU
Street Prepared: ASP
Street Modified: SM
Prepared: FP

The possible classes for a Forester XT are:
Street: N/A
Street Touring: STH
Street Prepared: FSP
Street Modified: SM

The possible classes for a Legacy 2.5GT are:
Street: GS
Street Touring: STH
Street Prepared: ESP
Street Modified: SM

One essential principle is common to all the rules: If a modification is not explicitly authorized by the rules, it is not allowed, no matter how trivial it seems.

Street Rules (BS, DS, GS)

The following modifications are allowed in street class:
Port installed options: rear spoiler, diff protector, armrest extension
Appearance, comfort and convenience options, e. g. gauges, alarms, turbo timers, shift knobs.
Removal of spare tire, tools and jack.
Harness (seat belts cannot be removed).
Any DOT approved tires with minimum 200tw rating(see rules for restrictions), of any size (no modification of fender well).
Wheels (+/-1" diameter from stock, offset within 0.25" of stock).
Wheel spacers (resulting offset within 0.25" of stock).
Lug nuts.
Brake pads, brake lines, speed bleeders.
Shocks that maintain stock ride height, can be adjustable. See rule book for details!
One, either front or rear, sway bar.
Different alignment using factory adjustments (no camber bolts).
Spark plugs.
Air filter element can be removed or replaced.
Cat-back exhaust.

STS Rules

All modifications from the Street rules are allowed, except for:
Tires only up to 225 width, and at least 200 tread wear rating.

In addition, the following modifications are allowed:
Remove air condition.
Pedal covers.
Seat (fully upholstered seating surface, must weight at least 15 lbs).
Steering wheel weighing at least as much as stock
Removal of factory emblems (debadging).
Rolling of inside fender lip.
Wheels up to 7.5" width, any diameter.
Cross drilled and/or slotted brake rotors (standard size).
Brake lines, e. g. stainless steel.
Air ducts for brakes.
Disabled ABS (not removed).
Shocks (fewer restrictions than stock). No electronic adjustment, like Tein EDFC!
Springs (can be height adjustable, e. g. coilovers).
Any suspension bushings, non-metallic.
Any differential mount bushings, non-metallic.
Any subframe mount bushings, non-metallic.
Adjustable camber plates (caster adjustment allowed).
Camber bolts.
Adjustable front control arms (only if no front camber plates or camber bolts are used). Must have non-metallic bushings, spherical bearings are not allowed.
Adjustable rear lateral links (only if no rear camber plates or camber bolts are used). Must have non-metallic bushings, spherical bearings are not allowed.
Both sway bars, mounts, end links
Strut bars (not triangulated).
Battery, relocation of battery (to trunk).
Electrical grounding cables
Air Intake.
Headers.
Reprogrammed ECU or piggyback that controls ignition and fuel by modifying inputs to ECU.
Short-throw shifter.
Crankshaft and accessory pulleys.
Clutch and pressure plate
Engine mounts (non-metallic).
Brakes with minimum stock outside rotor diameter(see rules for restrictions)

I lost steam, STS is not comprehensive, but is hopefully not incorrect? I'm not an expert on these rules. haha

Here is the most common for people who are actually competitive. IWSTI wasn’t bad but I rarely see anything there that’s good. Facebook tried, but good lord is the data murky or wrong most often.

There doesn’t seem to be a lot of newer Subaru people asking questions either.

I paid my SCCA dues this year and looking into some SCCA races. I'm currently racing AutoX Club at Limerock Park CT, it's a little different than SCCA parking lots. But this thread was very helpful.

The reason that not many people are running WRX lately is that it is not really competitive in DS class anymore when going against Civic Type R and others.

What’s the best way to get into autocross? Just show up to an event and observe?

Honestly, yeah. Or look for a local event and register as a novice. Ask lots of questions. Most people are pretty nice and helpful.

Boxerfest and the other Subiefests also have an autocross as part of the festivities. I will be doing course design at Boxerfest, and helping novices, etc. I'm not sure who my counterpart would be at the other events, but I'm sure there is someone.

Karen

You can get a ride along too if they have helmets to borrow (or you have your own) and you ask someone nicely.

Some events do online registration now (probably most actually) but a lot still have walk up registration if you want to try it out. Don't be afraid to ask them questions.

Where you are also helps because there are some regions that are worth a trip vs one that's a bit more local. I travel a few hours to go to a much larger and better site with more competition vs the local one that struggles to have 30 second courses.

But the others already said what's needed. Register as a novice, show up early, make sure you register on site first, then roll through tech with your numbers (likely painters tape) on the side and the car cleaned out. People should be friendly and answer all your questions, and if the region is large enough there should be people happy to ride along and help out as well.