Entry procedure and chances of getting a place for 2021 are very different than normal, but anyone wanting to enter the 2021 edition which is due to happen on 3rd October can enter the ballot here which is open until 5pm this Friday, 9th October 2020:
https://www.virginmoneylondonmarathon.com/how-to-enter/ballot-entry/index.html
There are ways of getting in via Good For Age, but unless you have already run the time, had a deferred place, and ran the virtual event last Sunday then chances are slim of getting in through that route. The number of place that will be available in the ballot isn't clear, not that it ever really is, but it's normally approximately 15-20,000 places for ballot and the same (ish) for charity.
I'd anticipate there being somewhere between 1/3 -2/3 of the normal number of places available though as people with 2020 entries will have been given a a place in one of the years 2021, 22 or 23. Not seen any rumours as to what they have done with charity places though, but that charities haven't been complaining on social media suggests they have been looked after so may have been issued a bunch of extra places that would normally go to ballot.
Championship places will be reduced by a bit next year as the times have been tightened by 5 minutes so that frees up some places, but makes GFA in subsequent years even tougher to obtain...but we don't know what GFA standards will be for 2022 or 2023 as yet.