With the world largely shut down in June 2020 due to the ongoing pandemic, Apple held its 2020 Worldwide Developers Conference digitally for the first time, allowing developers from all over the world to attend for free.
Apple's digital WWDC was a success, with the company offering online labs, developer sessions, and more, and the digital format continued for the rest of Apple's 2020 events.
Prior to when Apple cancelled the in-person component of WWDC 2020 and opted for a digital event, other major events were forced to cancel ahead of time, including E3, NAB, and others, giving us insight into Apple's plans at the time.
While there's now a vaccine, it does not appear that there will be a path to a large in-person event by the time June 2021 rolls around. As The Verge points out, San Diego Comic Con, Anime Expo, and E3 have all decided not to host physical events this year. Comic Con and Anime Expo are July events and often take place after WWDC, while E3 is a June event.
For those who were hoping that things might be back to normal enough to allow for a traditional WWDC event, 2021 will not be the year that happens, and Apple is likely to once again go with the same digital format that it used last year.
In many ways, Apple's digital format is more accessible because no one has to pay $1,599 for a ticket or travel to San Jose, California for the event itself, so it's quite possible that the digital format will continue on in some way even when physical gatherings are once again possible in the future.
It will probably be some time yet before we hear about Apple's official WWDC plans. Apple last year in March announced that WWDC would be a digital-only event, but it's not clear if we'll get the same early heads up this year. WWDC info last year went out on May 5, so we could see a similar timeline this year.