I found the following thread on the Silph Road worth sharing:
Shiny Scyther now appears to be boosted (Pineco/Aerodactyl rates?)
If they really have boosted rate now, what seems to be the case reading the responses there. It means they forgot to turn it on during the bug event. Nice work Niantic!!! 
I think your right 3 of my friends got one after the event
I have almost no Scythers spawning, Got 2 Aero’s. Not shiny. Pineco’s are so rare here.
There is alot of scythere out now and I got 2 shiny pineco now seem like we don’t need to look forward to the event but the days after the event lol
Quite sure they werent ment to be boosted in the first place, they never boost the rates
Probably just a coincidence
I know just saying
It makes more sense to have them boosted after the event due to the reduction in natural spawns unless you find a nest.
Event = Increased Spawns so no need for boosted rate.
No Event = infrequent/rare spawns so a slight boost in rate is warranted.
Sounds logical.
Usually the rate is increased. During an event catching 450 or more is undo-able for a normal player.
The encounter rate is, shiny rate stays the same as far as Im concerned
No as far as i know it is raised.
Definetly not a year ago, according to Silph Road
Will keep looking for a more up-to-date article, but the rates in our group seem to fit that 1/450 perfectly
I’ve seen other indications that it is raised on multipleday events.
They get raised on localised events, CDs, RDs and also the weekend it gets released (if it’s released in safari zone, like Roselia, Pinsir or Psyduck)
Sure. But is a normal player playing 5 accounts?
What do you mean?
The silph road post that was a basis for much of the speculation added together shiny rates from 5 accounts at the same scyther nest.
The combined total of those was closer to 550, though, and the 450 figure entered the conversation as just the denominator of the given rate, not an actual tabulation. So the numbers being thrown around seemed not to align all that well – neither with either, with theoretical rates, NOR with actual numbers (so make of it what you will, I guess).
I mean, how does a normal player not having 5 accounts change that 450 is unreachable for most people, sorry for not being exact 
Sorry I misunderstood the basis of your question. In which case I must inquire, why does 450 being unreachable during an event for most people have any bearing on the reported shiny rate?
Edit: Notice that none of the 5 accounts got to 450, either. The highest count was #3 with 178.
True, (s)he meant to support the claim that the rates are normally boosted (because Nia wants to give players a chance)