Here is a sort of list of events that happened since COVD (this is not 100% accurate, not an statement from an immigration office either, it's a "construction" from others members experience and mine experience.
1)All the local offices were shut down until October/November (i don't know exactly the date) (FIrst big bottleneck)
2) After that period the started basically to :
a) Already approved processes send to virtual oath once set.
c) Because the online testing came online into play at the end of November on a very limited basis, they started to process newer applications.
d)So, everybody that has the case analyzed started piling up.
3) In Late december/Early January they opened up the online testing for all the offices, and with that that all the case "ready" were sent for online testing federal centralized system (the local office don't send the invitation). This is second bottleneck, the backlog here is way over a month. From my experience and the little information I could obtained, this is a first-served basis line-up. So if your case was was "flagged" for some reason, the chances of you going to the bottom of the pile are huge.
IN my case, we were flagged by some form incorrectly used and the officer took the chance to already do the step 4) before (this is not the normal procedure); almost a month ago. we moved to the testing part and so far nothing. I've called IRCC last week, and I was told that the list is huge, and the waiting can last for months. nobody has priority and the resources to submit online testing invites are very limited.
4) After online testing, processes comeback again for final check on days calculation.
5) Here is the big thing with step 5. the invites for oath are being sent by the local offices, but i believe that the virtual ceremonies are federal. So my guess here there is another backlog, because federal has to notify the local office when the oath will take place and they need to send the invite.
I think that is why we are starting to see a lot of 2020 for testing already. probably the will have some lag coming back to the local offices.