OP I’m hoping this response will be a bit more thoughtful and a bit less judgmental than some of those upthread.
I used to teach (primary including Early Years), have an Master’s in an area allied to child development and am considering retraining in speech and language therapy, so know a little bit about this. I also have an almost 4 year old DD.
Personally I dislike masks, and instinctively seek to avoid wearing them around my own child. But with masks currently mandated only for shops and public transport I wouldn’t be unduly worried about any developmental impact; I get that it’s horrible not being able to chat to your little one unhindered, but as others have said most early language acquisition happens in other contexts. I’d be somewhat more concerned if the mandate is extended to other settings, such as toddler groups and indoor leisure activities, or if caregivers in childcare settings were required to wear masks.
With DD we’ve recently been visiting potential primary schools, and every one of the five we visited mentioned that they’d noticed an increase in early speech and language and socialisation issues this year. Mostly this was attributed to lockdown, but the staff I chatted to did think mask wearing had had some impact too. More children were presenting with S&L difficulties prior to Covid anyway due to screen use, lack of quality communication at home etc - I saw a lot of this when I was teaching - and for vulnerable children in particular it doesn’t take much to tip them over the edge into it becoming a diagnosable problem.
The most important thing is that you’re interacting meaningfully with your DS at home, modelling good language use, not letting him have excessive screen time, and providing opportunities for him to observe and communicate with a variety of people in a range of settings when out and about. I think a lot of the issues that schools are seeing now stem from the very limited range of experiences some of these children were exposed to as a result of lockdown measures. Make sure he’s taken to cafes, markets, toddler groups, swimming, parks - as many settings as you can reasonably manage.
Apologies for the long post but hope it’s all vaguely reassuring. 