My grandson was born during 2020 and shortly after his birth, his parents moved to Germany for just under 2 years as my son was working at a German university. The family experienced German lockdown which made it difficult to make friends and they were also dealing with the language barrier as neither DS or DiL were fluent in German. They returned to the UK when GS was 21 months and he was very dependant on mummy. However he attended nursery soon after arriving in the UK and has become very balanced socially. His mum is training as a vicar so he meets a wide variety of people at church, and as the curate's son everyone knows him and he mixes well. His younger sister who is nearly 2 now is much less 'mummy focussed' so I see the difference, but I don't think he has been badly held back as he is developing well socially.
I think the children who have been most affected are those who were isolated after starting school as they missed their friends, and how much home schooling they had depended very much on their parents circumstances.
At the time of lockdown I worked as a Learning Support Assistant in a secondary school. Special needs children received doorstep visits from members of our team, but most children had only emailed work set. By the second lockdown schools had improved their online offerings to include actual lessons delivered on 'Teams' with support workers attending online, but I had retired by then so don't know how well that worked. I was relieved not to have to negotiate that as my technical skills leave a lot to be desired! Sadly the more deprived the family, the more they were affected, as their parents were often unable to give them the support they needed, especially where they were having to work from home themselves and may not have had enough electronic devices for everyone to use. Although schools were open for children of essential workers, only about a dozen of our pupils were attending from a very large secondary school! Many more children were eligible for that so I don't know why more did not use the facility. It was staffed by a couple of teachers and a few Learning Support Assistants on a rota but I only did one session, as initially the school classified me as vulnerable! When they did eventually agree to let me go in, the summer holidays were about to start! We spent most of our time trying to enforce mask wearing and social distancing and not much time helping with the work!