Thank you for the hint, @explit! Is F-Droid Legacy the same as F-Droid Classic?
Now I’ve got F-Droid Classic on the phone, but initially, it couldn’t load any package indexes out-of-the box; the error message is “Error getting index file” – even if I only select the package source “F-Droid Classic”.
This was a problem with the CA trust list, which I could fix by installing the Let’s Encrypt root certificate with the help of Root cert problems on AppSupport 4.4.4 - #2 by ugeuder. Most package sources were usable then, except “F-Droid Archive” (app crashes because archive seems to be to big) and “F-Droid” itself, which uses another root CA (HARICA TLS ECC Root CA 2021). I could have inserted that other root in the same way as with Let’s Encrypt, but I thought, I should get the complete bunch of relevant root certificates at once. Indeed, I found a solution for this: Update root CA certificates for legacy Android AppSupport system - #2 by Chwinterhoch5.