I guess I should chime in with our recent flights also. I called both airlines to add allergies to my file and request buffer zones, as permitted.
Air Canada
We chose not to preboard on any of our flights since we were already in one of the first groups to get on the plane.
Flight #1 - flight attendant came to ask about my allergies, if I had EpiPens, and made a buffer zone announcement. Great.
Flight #2 - flight attendant just made a buffer zone announcement without checking with me (though I ordered a buffer zone ahead of time; they only asked people not to consume peanuts and cashews though, which was super weird and wrong.)
Flight #3 - nothing: no check, no buffer zone announcement, nothing. I reminded them eventually, but even then they didn't make the right buffer zone announcement again, just saying peanuts this time after I specifically said all nuts and peanuts.
Flight #4 - nothing again!! It was a super short 11-minute flight, so I didn't mention it. Plus, we had a disturbing experience at the gate that threw everyone off their game, so I don't blame them for this one. I will give the gate agent kudos for noticing my husband and I were seated separately (10 rows apart) and calling us up to seat us together, as she wanted me to have my husband nearby if I needed assistance (in case of reaction) - so that was really nice.
United
Flight #1 - no accommodations made other than they said they would let me preboard if I wanted to
Flight #2 - same as above
Verdict: I was less than pleased with Air Canada's allergy protocols this trip. I could have put more effort on my end, certainly, to prompt the flight attendants to do things right, but I guess I'm so used to WestJet doing things so well I wasn't on top of it. (Plus, I actually of wanted to test Air Canada for my clients' sake to see what their allergy protocols are like for people who may not prompt everything to happen as it should.) It's not that they did a poor job, but Air Canada could have done better without my prompting. As for United, I didn't expect anything and wasn't told they would make an announcement, so I got what I expected, much like when flying American, only the United phone agent was far more organised when noting the allergies on my file.
We normally fly WestJet exclusively, and we'll be sticking with them whenever possible, for sure; there's no question they have better allergy policies than Air Canada does. I'll be booking my clients on WestJet whenever possible too.