Solution:
I think you need something like the following.
RewriteRule ^/\d+/\d+/\d+/(.*)$ https://www.domain.xyz/$1 [R=301,L]
// ^/ start at the root
// \d+/\d+/\d+/ match date folders like 2020/03/25/
// (.*) the part we want to keep “some-slug”
// $ end of match
// $1 put the part we want to keep here “some-slug”
Basically matching any urls with /2020/03/25/post-name and redirects to https://www.domain.xyz/post-name.
Note: this assumes you’re adding the redirect at the old domain.