My experience with the new tax portal
Pandemic or not, taxes still have to be paid. Some time ago I set up a tax information mailbox (DIS+) on the Moje daně portal. Login is standard by today's norms (NIA, Data Box, or credentials from the Financial Administration). The portal has a modern look and easy navigation.

After clicking into your account you get to the electronic forms for individual income tax returns, with two to choose from.

The first, newer one will work for most people: Income tax return for individuals with income only from employment in the Czech Republic (including Czech tax non-residents) — from 2016 onwards. Everyone else will have to make do with the familiar old form.

This form has a new design and is easy to fill in. Unfortunately not for Safari users — as soon as you click somewhere, you can't click elsewhere to fill in other fields on the same page. You can work around it by going back to the previous page, but that's a hack, not a solution. I get the feeling the new tax portal is really only suited for people who have income solely from employment, use Windows, and browse with Google Chrome. I tried it and it also works fine on macOS in Chrome.
The Financial Administration should be embarrassed — the whole solution isn't actually new. It's just a new skin on the old system. ADIS, which dates from the 1990s, is still running underneath!