As a new feature,Plesk picked up zstd compression method for backups.If you want to create backup files with classic method you can edit the panel.ini file in two way

1. Go to Extensions ->Install and activate panel editor extension and open the editor in my extensions section

Select the editor and add these;

[pmm]
compressionMethod = deflate

2.Login to shell.Open /usr/local/psa/admin/conf/panel.ini file with your favorite editor like nano or vi.Add theses line to the appropriate place

[pmm]
compressionMethod = deflate

Note.You may already see [pmm] section.In this case you just add compressionMethod = deflate

below the [pmm] section.