Is Anmeldung possible without an appointment?

The rules differ from office to office, but in general walk-in appointments are not possible currently.
But by having an appointment later, you are absolved from the requirement to register within a week of moving.

I would add that Covid-19 infection rates are currently way up in many parts of Germany. I just read a notice on the site for the Stuttgart Bürgerburos that the rate of over 50/100,000 has been broken and as of tomorrow Oct. 14th there are automatically strict new measures going into place.

Most such government offices, if they weren’t by appointment only already, are likely to soon be so.  But such offices should have a  tel. number to be found on their websites where one can call and double check.