There is ONE thing that reduces pollution.
Fewer cars (or fewer journeys, depending on how you phrase it).
LTNs help with that by deterring (some) car use. It takes time to bed in, it sometimes requires additional interventions like setting up bike or scooter hire schemes or better parking for them, Walking Buses for kids to get to school, pop-up cycle lanes, 20mph zones and so on, sometimes it'll largely achieve the desired effect on its own.
The roads aren't closed
- residents, emergency services, deliveries etc can all reach all houses and businesses so it's an important phrasing distinction between "closed road" and "filtered road" (where through traffic is prohibited).
We're in a situation which was going to arise sooner or later, Covid or not. There's a climate emergency, there's a global pandemic, there's an obesity and diabetes crisis especially amongst children and it's got to the point where we could have made minor changes over the last 20 years, drip fed into the system but we haven't. It's now at the point where it needs dramatic intervention NOW because, if we don't do it, it'll need even further dramatic intervention next year (like the Athens case mentioned above where odd and even-numbered cars were banned from the streets on alternate days - happened in Paris as well: [www.theguardian.com
The choice now is you can be sort of nudged into making fewer journeys because it's mildly inconvenient to drive or you can wait a year or so and be banned from driving on alternate days altogether.
If you're angry about pollution, look to the people driving short journeys, not the LTNs.