I get the impression the "all tournament style all the time" dynamic was more true in the past and maybe it's still true in some places but GW have done great work in supporting Narrative (Crusade got a *ton* of stuff in 9th and a launch book for 10th) and non-tournament Matched Play (to the extent the excellent Tempest of War actually invaded tournament play for 10th, not the other way around

Also like Dave said there's question marks over how much the vocal internet minority actually plays. Tournament players tend to be more chill IME, or at least worked up about different issues

I haven't seen anything in Legends that puts it anywhere near the salience of "playing Aeldari, Custodes, Knights or a Primarch". I tried my 30K list online and it wasn't bad but nowhere near the best Marine list I could put together.
I'd fuzzily say the etiquette should be:
1: Don't play an overtuned list without prior arrangement (I've been guilty of this when thinking "oh, Custodes are fine right?"

2: Do your best to get the terrain right (if there's anywhere where you want to copy tournament play, the GW layouts are probably it)
...long list of basic courtesies, aisatsu, don't use the worst faction dice you can find for real rolls Jye, being packed up by 4:30...
XXth: Ask about Legends
One good habit I think is probably bringing some substitutes for the spicier units in your list so you can tone down a bit. I think I'll do that for this month - Bobby G is a really powerful force multiplier who can elevate a middling Marine list so even just being able to easily swap him and out would help me pitch a fairer game.