hey there ​@Ivan Hauser 

track availability is a decision from artists and publishers, and not deezer. sometimes it’s a temporary thing, sometimes it’s permanent.

deezer is my preferred service and i am impressed with their catalog. if, however, despite the tens of millions of tracks in the catalog, there are songs that become unavailable, might i suggest purchasing them? that way you own the music, are supporting your favorite artists, and can upload it to your deezer account. here is how: https://support.deezer.com/hc/en-gb/articles/115004221605-Upload-MP3s-To-Your-Deezer-Account

thank you very much