How long will an M1 2020 MacBook Pro receive updates for?

When a new MacOS is released, sometimes the new macOS is too intensive or has standard features that the old Macs just can't handle, so they are not eligible for the new Upgrade. Those left-behind models can still do everything they could do the day before the upgrade was released -- it's not as though they fall over dead!

Iif your Mac (2017 and older in this case at this writing) will continue to get security updates for TWO more releases after you can no longer Upgrade to the new System.

So although 2017 can not run Sonoma, 2017 model will still get a security upgrade, and will be eligible for one last security upgrade when Sonoma's successor is released, expected a year-ish later.

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That said, there is no accurate way to say when the 2020 MacBook Pro models will be on the "can't be upgraded to the new MacOS" list. Whenever that is, you will likely have two more years before Security Upgrades are discontinued.

Unknown. Apple does not really publish dates for end of Software Update support.

It ends when the device can no longer properly run the new operating system and there's no way to know that until a version is made that it can no longer run.

But extrapolating from the 2017 MacBook Pro, that's around 6 to 7 years of updates, so the 2020 MacBook Pro may probably receive updates until around 2026 or 2027.