This is done automatically. All strings deleted from the source files will be moved to the project recycle bin with its translations. The recycle bin can be used as translation memory sourcewhen doing a pre-translation.

So are the translations stored there permanently if you don't delete them manually?

Yes, in the default settings the recycle bin is activated and will store all these items permanently. You can switch off the usage or delete the recycle bin manually.

After discussing that with some colleagues, we are not too happy with this. 

Isn't there any other way to store these translations? We are just scared we lose them, since recycle bin doesn't really sound like a translation memory. It just feels that we can not guarantee that it will be deleted some day.

Thanks again

There are many ways to store „deleted“ translations.

Clients working not exclusively in SDL Passolo usually store UI strings in an additional external translation memory. Passolo offers functions to directly store translations into external TMs or export these translations. The export options can include deleted strings from the recycle bin.

Why does recycle bin sound scary? The use case for the recycle bin is exactly the described scenario: Keep translation for UI string entries deleted by developers. And it can be used as a translation memory within SDL Passolo

And in the end you can’t guarantee that these entries will not be deleted from any other repository where you transferred the deleted strings.

Anyway there is also another viewpoint. I know clients that do not store or reuse the translations of deleted string entries as the engineering effort to maintain this process is more expensive than letting the translation vendor retranslate some strings.